The moment of instability for internal solitary waves

Abstract: In this note, we define a moment of instability m(c) for internal solitary waves in continuously stratified fluids, which seems not to have been done before. To underline the suitability of the proposed moment of instability, we identify the relation m"(c)=0 as a formal Fredholm condition, and we show that m(c) displays a definite sign for small-amplitude waves.