Function: poissubst (<a>, <b>, <c>) Substitutes <a> for <b> in <c>. <c> is a Poisson series.
(1) Where <B> is a variable <u>, <v>, <w>, <x>, <y>, or <z>, then <a> must be an expression linear in those variables (e.g.,
6*u + 4*v).
(2) Where <b> is other than those variables, then <a> must also be free of those variables, and furthermore, free of sines or cosines.
poissubst (<a>, <b>, <c>, <d>, <n>) is a special type of substitution which operates on <a> and <b> as in type (1) above, but where <d> is a Poisson series, expands
sin(<d>) to order <n> so as to provide the result of substituting
<a> + <d> for <b> in <c>. The idea is that <d> is an expansion in terms of a small parameter. For example,
poissubst (u, v, cos(v), %e, 3) yields
cos(u)*(1 - %e^2/2) -.
sin(u)*(%e - %e^3/6)
(%o1) true (%i2)