Quadratic addition rules for three $q$-integers

Abstract: The $q$-integer is the polynomial $[n]q = 1 + q + q^2 + \dots + q^{n-1}$. For every sequences of polynomials $\mathcal S = {s_m(q)}{m=1}^\infty$, $\mathcal T = {t_m(q)}{m=1}^\infty$, $\mathcal U = {u_m(q)}{m=1}^\infty$ and $\mathcal V = {v_m(q)}{m=1}^\infty$, define an addition rule for three $q$-integers by $$\oplus{\mathcal S,\mathcal T,\mathcal U,\mathcal V} ([m]q, [n]_q, [k]_q) = s_m (q) [m]_q + t_m (q) [n]_q + u_m(q) [k]_q + v_m (q) [n]_q [k]_q .$$ This is called the first kind of quadratic addition rule for three $q$-integers, if $$\oplus{\mathcal S,\mathcal T,\mathcal U,\mathcal V} ([m]q, [n]_q, [k]_q) = \left[m+n+k\right]_q$$ for all positive integers $m$, $n$, $k$. In this paper the first kind of quadratic addition rules for three $q$-integers are determined when $s_m(q)\equiv 1$. Moreover, the solution of the functional equation for a sequence of polynomials ${f_n(q)}{n=1}^\infty$ given by $$f_{m+n+k} (q) = f_m (q) + q^m f_n (q) + q^m f_k (q) + q^m (q-1) f_n (q) f_k (q)$$ for all positive integers $m$, $n$, $k$, are computed.