 Let the restriction of formula_3 to the level set formula_4 for formula_5 a regular value, be called formula_6; it is a Morse function.Then the Jacobi set formula_7 of formula_8 and formula_9 is formula_10,

Alternatively, the Jacobi set is the collection of points where the gradients of the functions align with each other or one of the gradients vanish (cite?