odeef.spad line 502 [edit on github]
ReductionOfOrder provides functions for reducing the order of linear ordinary differential equations once some solutions are known.
ReduceOrder(op, s) returns op1 such that for any solution z of op1 z = 0, y = s \int z is a solution of op y = 0. s must satisfy op s = 0.
ReduceOrder(op, [f1, ..., fk]) returns [op1, [g1, ..., gk]] such that for any solution z of op1 z = 0, y = gk \int(g_k-1 \int(... \int(g1 \int z)...)) is a solution of op y = 0. Each must satisfy fiop .fi = 0