$\exists \mathbb{R} \subseteq \textsf{CH}$

Abstract: The existential theory of the reals asks whether polynomial constraints with integer coefficients have a real solution. We give a proof placing this problem in the counting hierarchy. The first argument is intended to expose the essential steps, and a separate analysis lowers the bound to $$\exists \mathbb{R}\subseteq\textsf{BPP}^{\textsf C_3\textsf P}\subseteq\textsf C_4\textsf P$$, the fourth level of the hierarchy. For each fixed $w$, sentences with $w$ alternating real quantifier blocks lie in $\textsf C_{9w+17}\textsf P$. We then treat exact semidefinite feasibility, PosSLP, square-root sum, geometric real counting, Euler characteristic, and complex feasibility in separate applications. The corresponding bounds include $\textsf{BPP}^{\textsf C_2\textsf P}$ for general SDP, $$\textsf{BPP}<sup>{\textsf{PP}}\cap\textsf{P}<sup>{\textsf{NP}<sup>{\textsf{PP}}}$$ for PosSLP and square-root sum, and $\textsf{FP}^{\textsf C_4\textsf P}$ for total geometric real counting. Note: These proofs were discovered by ChatGPT after a series of conversations ending on September 29th 2026. A group of researchers has been working to digest the proof, and while the most essential arguments appear correct, we are endeavoring to give this result the treatment it deserves and a proper exposition and development to benefit of the community. However, on October 6th, OpenAI released a very similar result, with a slightly weaker bound. While we work to improve our exposition of this proof, the current version has been uploaded as a service to the community to compare the different proof techniques. While the listed author takes responsibility that the proofs appear to be correct, he has not played a nontrivial role in developing them, and believes the human value will be in good exposition and canonicalization of the results.

• This paper investigates the existential theory of the reals ($\exists\mathbb{R}$), providing a comprehensive complexity analysis of its feasibility problem showing $\exists\mathbb{R}${}\subseteq${}$\mathsf{MP}^\mathsf{#P}_{3}$\subseteq${}$\#\mathsf{P}_{4}$$
• The real feasibility problem is reduced to the sign of an exponentially large integer without needing an explicit construction, using modular trace computations and numerical perturbations to manage complexity.
• The manuscript provides novel techniques for trace computation without multiplication matrices, relying instead on residue-based methods and polynomial arithmetic to enable efficient evaluations.

Central result and scope

The manuscript studies the complexity of the existential theory of the reals, $\exists\mathbb{R}$, whose instances are finite systems of polynomial equations and inequalities with integer coefficients and whose witnesses are real tuples. Its principal unconditional result is

$$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$

where the manuscript uses $\#\mathsf{P}_{0}=\mathsf{P}$ and $\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$. It also gives a more direct but less tightly accounted-for containment $\exists\mathbb{R}\subseteq\mathsf{CH}$, a fixed-block real quantifier bound

$\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$

and a collection of consequences for exact semidefinite feasibility, $PosSLP$, square-root sum, geometric real counting, Euler characteristic, and complex feasibility (2610.10514).

The central methodological claim is that real feasibility can be reduced to the sign of an implicitly represented, exponentially large integer. The integer is not constructed explicitly. Instead, the proof combines a finite critical-point algebra, residue-based trace computation, univariate characteristic polynomials and resultants, modular evaluation through a common formal homotopy, and randomized sign reconstruction. The manuscript’s principal technical contribution is therefore not a new algebraic-geometric reduction from feasibility to critical points by itself, but an oracle-complexity analysis of how the resulting algebraic objects can be accessed without expanding their exponential dimension.

From arbitrary feasibility to a finite critical algebra

The first reduction replaces an arbitrary semialgebraic feasibility problem by the existence of a zero of a coercive polynomial. Sign conditions are encoded using auxiliary variables, yielding integer quadratic equations $f_i(x)=0$. A large dyadic bound $B$ is supplied by effective real-algebraic sampling, with the property that every feasible instance has a solution in a bounded box. The construction then forms

$$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$

where $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$0 is chosen from the bounding radius. Thus $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$1, $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$2 is coercive, and the original instance is feasible exactly when $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$3 has a real zero.

A height argument establishes a crucial quantitative gap: if $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$4 has no zero, then

$$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$5

for all real $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$6, where $$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$7 has a polynomial-size arithmetic description despite potentially having exponentially many bits. The proof introduces a sextic perturbation and studies critical values through a determinant polynomial. Importantly, the determinant is used only to prove an algebraic height bound; it is not computed by the algorithm.

The perturbed objective is

$$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$8

Its critical equations have the form

$$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$9

Because the leading monomials are pairwise coprime pure powers, the quotient algebra

$\#\mathsf{P}_{0}=\mathsf{P}$0

has the explicit monomial basis $\#\mathsf{P}_{0}=\mathsf{P}$1 with $\#\mathsf{P}_{0}=\mathsf{P}$2 and dimension $\#\mathsf{P}_{0}=\mathsf{P}$3. This finite algebra may be nonreduced; the manuscript deliberately retains multiplicities rather than assuming generic nonsingularity. Feasibility is reduced to the existence of a real point in this algebra satisfying a sign condition $\#\mathsf{P}_{0}=\mathsf{P}$4.

This reduction avoids the usual requirement that a feasible point possess a short rational or algebraic certificate. Its implication is that the difficulty of real feasibility is transferred from finding a witness to recognizing real points in a structured finite algebra whose rank is exponential but whose monomial representation has polynomial-length addresses.

Traces without multiplication matrices

The algorithm never constructs the $\#\mathsf{P}_{0}=\mathsf{P}$5 multiplication matrices explicitly. Instead, it computes their traces through a global residue identity. For equations of the form

$\#\mathsf{P}_{0}=\mathsf{P}$6

the trace of multiplication by $\#\mathsf{P}_{0}=\mathsf{P}$7 is extracted as a single coefficient of

$\#\mathsf{P}_{0}=\mathsf{P}$8

where $\#\mathsf{P}_{0}=\mathsf{P}$9 is the Jacobian determinant. The finite geometric sums have logarithmic-depth arithmetic circuits, and the relevant coefficients are accessible through Kronecker substitution and the BitSLP theorem.

Applying this identity to powers of a separating linear form $\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$0 yields the power traces

$\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$1

A second family,

$\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$2

encodes the sign predicate. Newton identities would recover characteristic-polynomial coefficients sequentially, which would introduce an undesirable depth proportional to the algebraic rank. The manuscript instead reconstructs the characteristic polynomial by a truncated formal exponential:

$\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$3

After clearing denominators, parallel polynomial arithmetic recovers all coefficients in uniform threshold-circuit depth. This is a key implementation choice: the manuscript’s complexity bounds depend on replacing sequential algebraic reconstruction by fixed-depth packed arithmetic.

The resulting polynomials are

$\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$4

and

$\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$5

At a point $\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$6 with projected value $\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$7 and local multiplicity $\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$8, the manuscript derives

$\#\mathsf{P}_{h+1}=\mathsf{PP}^{\#\mathsf{P}_{h}}$9

and

$\exists\mathbb{R}\subseteq\mathsf{CH}$0

Consequently, for each multiplicity stratum, the sign of

$\exists\mathbb{R}\subseteq\mathsf{CH}$1

encodes the sign of $\exists\mathbb{R}\subseteq\mathsf{CH}$2. This handles repeated and singular fibers without perturbing the algebra into a reduced one.

Selecting a real root using a short sign description

The characteristic polynomial contains complex roots as well as real roots. The real-feasibility problem therefore requires a root-selection mechanism that distinguishes real roots satisfying the desired inequalities.

The manuscript uses Thom sign vectors: the signs of the derivatives of a univariate polynomial at a simple real root identify that root. A collection of at most $\exists\mathbb{R}\subseteq\mathsf{CH}$3 ternary sign vectors can be isolated by fixing at most $\exists\mathbb{R}\subseteq\mathsf{CH}$4 coordinates. Thus a polynomial-length guessed list of derivative indices and signs suffices to isolate one qualifying root, even though the full sign vector may have exponential length.

To turn this list into an integer sign test, the proof constructs a rational sign approximation $\exists\mathbb{R}\subseteq\mathsf{CH}$5 with positive denominator on the real line. The approximation preserves the sign of all relevant nonzero algebraic values and has error small enough that a polynomial selector

$\exists\mathbb{R}\subseteq\mathsf{CH}$6

is negative precisely at the selected qualifying roots. Clearing denominators produces an integer polynomial whose sign pattern is correct on the real roots of $\exists\mathbb{R}\subseteq\mathsf{CH}$7.

The selector may vanish at nonreal roots. The manuscript removes this problem by shifting it by a sufficiently large positive integer and taking a resultant. Nonreal conjugate roots contribute positive factors, while each selected qualifying real root contributes one negative factor. Therefore the sign of the norm is

$\exists\mathbb{R}\subseteq\mathsf{CH}$8

A guessed list that isolates one qualifying root makes this sign negative. Conversely, a negative norm certifies the existence of a real point satisfying the original condition. This establishes the high-level containment in $\exists\mathbb{R}\subseteq\mathsf{CH}$9.

The conceptual implication is important: realness is recovered not by explicitly isolating algebraic numbers, but by encoding the parity of selected real roots into the sign of an integer norm.

The fourth-level refinement

The sharper bound does not follow merely by assigning an explicit depth to the first proof. The manuscript changes the algebraic construction and the root-selection mechanism.

First, it reduces to a bounded ETR form involving variables in $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$0 and equations such as $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$1, $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$2, and $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$3. A weighted sextic perturbation produces critical equations whose reductions modulo two are copies of

$\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$4

The polynomial factors as

$\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$5

over $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$6 and has five distinct roots over $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$7. Hensel lifting therefore yields $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$8 simple characteristic-zero critical points. The carefully chosen two-adic weights ensure that all critical values are distinct, and the threshold is not itself a critical value.

This eliminates the multiplicity stratification required by the first proof. The projected critical-value polynomial is squarefree, and its real roots correspond directly to real critical points.

Second, the proof replaces full Thom vectors by ordered integer labels for real roots. A randomized rational normalizer is designed so that, with high probability, the signs of transformed differences are ordered along each horizontal line. For a real root $\Sigma_w\cup\Pi_w\subseteq \#\mathsf{P}_{9w+17},$9, the sum of pairwise signs produces a label

$PosSLP$0

which is an integer and strictly increases from left to right among the real roots. The labels are not claimed to preserve the original complex half-plane ordering globally; only the monotonicity required for real-root ranking is established.

The labels are computed through a first-order jet of a norm:

$PosSLP$1

The ratio $PosSLP$2 approximates the rank at a real root. Its bits are then used in a random affine hash. With constant probability, exactly one qualifying label is isolated. A selector norm is negative exactly when an odd number of qualifying roots falls into the selected hash bucket.

The arithmetic is organized around one shared formal homotopy. Formal branches from a split reference system are continued to the target critical system using polynomially many Newton updates. The same branch family is reused for the inner rank norm and the final selector norm. Products over exponentially many branches are evaluated over finite rings, and coefficient extraction is performed by Fourier identities. The final integer is represented by raw modular counting expressions rather than being canonically reduced before the next counting operation.

The resulting accounting is

$PosSLP$3

The manuscript emphasizes that the last inclusion is a consequence of the relativized inclusion $PosSLP$4, not a collapse of the counting hierarchy.

Modular sign reconstruction

A central difficulty is that the final norm can have exponentially many bits. The manuscript develops two sign-reconstruction methods.

The deterministic method uses the identity

$PosSLP$5

If $PosSLP$6 is the hidden integer, modular representatives of $PosSLP$7 and of the binomial coefficients are combined into a rational number

$PosSLP$8

where $PosSLP$9 is an integer and $f_i(x)=0$0 is exponentially small but has a known lower bound when $f_i(x)=0$1. Repeatedly doubling the displacement $f_i(x)=0$2 modulo one eventually produces a first scale at which the fractional part moves a fixed distance from an integer. The direction of this first event determines the sign of $f_i(x)=0$3.

For $f_i(x)=0$4, the modular representatives are directly computable. The resulting deterministic bound is

$f_i(x)=0$5

For the ETR norm, the modular computation is more deeply nested and gives an intermediate bound before the randomized refinement.

The randomized method replaces the first-event search by random phase testing. A short-seed generator produces the bits of an exponentially long random multiplier while supporting random access to individual bits. The expected cosine of the resulting phase distinguishes the regime where the displacement is small from the regime where it has become visible. Binary search over scales locates a suitable phase magnitude, after which one deterministic phase query reveals the sign.

This yields

$f_i(x)=0$6

and, after applying the method to the ETR norm with its nested modular computation,

$f_i(x)=0$7

before the ordered-rank refinement, and finally the stated fourth-level bound.

The paper is explicit about a limitation of this approach: modular residues alone cannot be naively aggregated into one universal counting gap that decides sign over a doubly exponential range. The successful reconstruction depends on nonlinear postprocessing after retrieving or coherently combining modular information.

Fixed real quantifier blocks

For a fixed number $f_i(x)=0$8 of alternating real quantifier blocks, the manuscript obtains

$f_i(x)=0$9

The proof addresses a problem absent from the existential case: after choosing an outer real sample, the coefficients of an inner formula become algebraic numbers, and later predicates must refer coherently to the same earlier sample. A Boolean ETR oracle is insufficient because independently guessed algebraic samples need not represent the same root branch.

The proposed representation consists of a defining polynomial, a selected real root, and rational coordinate functions of that root. Validity includes denominator guards and degree/gcd metadata so that roots do not split, collide, or acquire new zeros within a connected parameter cell. Root branches are addressed by hashed Thom data, and one common infinitesimal specialization is used for the entire sample tree.

The explicit level ledger charges nine counting levels per real quantifier block, followed by fixed costs for recoding tuples, guarded resultants, sign reconstruction, and Boolean alternation:

$B$0

The coefficient is explicitly described as conservative rather than optimal. The theorem is only for fixed $B$1; it does not place formulas with input-dependent quantifier depth at one fixed level of $B$2.

Consequences for related problems

The manuscript derives several bounds by applying the same arithmetic interfaces to different geometric or algebraic structures.

For exact rational semidefinite feasibility, both general and strict feasibility satisfy

$B$3

The argument uses the fact that a nonempty compact convex semialgebraic set has Euler characteristic one, whereas the empty set has Euler characteristic zero. PSD constraints are encoded by nonnegativity of the coefficients of the characteristic polynomial of a symmetric pencil. The result is unconditional and does not assume a numerical gap.

For $B$4 and specified coefficient positivity, the manuscript claims

$B$5

The paper carefully distinguishes this from a claim that $B$6 lies in $B$7 or in $B$8. It also notes that modular sign reconstruction for the ETR norm is more complicated than for an SLP because the nested norm computation involves nonlinear operations on counting outputs.

The square-root-sum problem inherits

$B$9

through its standard reduction to $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$0. The paper does not establish the stronger certificate-based possibilities such as $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$1.

For geometric real counting, the total threshold problem for the number of distinct points is placed in $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$2, yielding

$$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$3

The construction uses a signed Morse count. A random address selects critical points, and the acceptance bias is proportional to the Euler characteristic or cardinality being measured. The infinity case is separated using an effective lower bound on the distance between distinct points of a finite semialgebraic set.

The paper further claims that low-order bits of the Euler characteristic of arbitrary Boolean semialgebraic sets are computable in $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$4. The proof uses Morse-theoretic critical-point counts and Lucas’s identity

$$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$5

to recover logarithmically many bits through parity computations.

Finally, complex feasibility and complex dimension are placed in

$$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$6

using a separate resultant and homotopy construction based on prior work. This application does not depend on the real-root selection part of the ETR proof.

Limitations and open questions

The manuscript’s strongest claims depend on several technically delicate interfaces. The most important limitation is the distinction between a short arithmetic circuit for an individual local computation and a short description of the entire exponentially large norm. The proofs frequently establish height bounds or coefficient-access predicates without producing a polynomial-size SLP for the final integer. The text explicitly warns that a determinant or norm having a succinct mathematical definition does not imply that its value has a polynomial-size straight-line program.

The fourth-level ETR bound also depends on shared formal homotopies, uniform degree bounds, arbitrary prime-power arithmetic, and raw counting representations that remain coherent through nested products. Canonical residue extraction at an intermediate stage could introduce an additional counting level, so the order of retrieval and nonlinear postprocessing is essential.

The paper develops several conditional certificate routes based on sums of squares, conic identities, trace-form isometries, and coefficient-defined polynomial families. These routes would imply bounds such as $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$7 or lower, but the manuscript does not prove polynomial-size descriptions for the required dense algebraic numerators. The limitation is not existence: finite-dimensional real algebras admit relevant sum-of-squares representations. The unresolved issue is succinct representation.

Several claims are also explicitly nonuniform with respect to a fixed number of real quantifier blocks. The bound $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$8 grows linearly with $$F(x)=\sum_i f_i(x)^2+\bigl(\lVert x\rVert^2-R^2\bigr)^2,$$9, and no input-dependent quantifier-depth collapse is claimed. Likewise, the numerous arithmetic extensions—root gates, sparse roots, monotone inverse gates, stochastic systems, and certificate promises—do not provide reductions from unrestricted ETR to those restricted models.

Conclusion

The manuscript presents a unified complexity framework for real-algebraic feasibility based on critical-point algebras, residue traces, formal homotopies, norm signs, and modular reconstruction. Its principal unconditional claim is

$$\exists\mathbb{R}\subseteq \mathsf{BPP}^{\#\mathsf{P}_{3}} \subseteq \#\mathsf{P}_{4} \subseteq \mathsf{CH},$$00

supported by a separate direct containment in the counting hierarchy and by a linear bound for every fixed number of real quantifier blocks. The technical significance lies in preserving real-root information through complex algebraic computations while controlling exponential dimensions through indexed arithmetic rather than explicit expansion. The remaining questions concern whether the nested norm computations and positive certificates admit substantially more succinct representations than those established by the current modular constructions (2610.10514).