# DSolve breaks when the ordering of independent variables aren't proper?

Bug introduced in 3.0 or earlier and persisting through 12.0.

I encountered this when trying to solve this problem with DSolve:

eqn = With[{p = p[x, y, t]}, D[p, t] == x D[p, x] + (1 - y) D[p, y] + 2 p];
DSolve[eqn, p, {t, x, y}]


DSolve::deqx: Supplied equations are not differential equations of the given functions.

(* Input returned *)


It's not surprising to see DSolve fails in solving PDEs… but why this warning? Even more shocking:

DSolve[eqn, p, {x, y, t}]

{{p -> Function[{x, y, t}, C[1][x (-1 + y), t + Log[x]]/x^2]}}


The result changed just because I reordered the independent variables (from {t, x, y} to {x, y, t}). Is this a bug?

• Very odd indeed; on mathematica 10 the first request is returned unevaluated. – chris Jul 22 '14 at 7:29
• With Mathematica 5.2 I get exactly the same result (screenshot) as with versions 8.0.4 and 10.4 (OS Windows 7 x64). So the bug seems to be quite old... – Alexey Popkov Apr 17 '16 at 15:52
• @AlexeyPopkov The bug has already been introduced in v3: i.stack.imgur.com/Zx2uD.png Version 2.1 isn't influenced, because DSolve can't seem to solve PDE at that time: i.stack.imgur.com/V1Se1.png – xzczd Dec 12 '19 at 4:34
• Yes, the order matter but as you noticed, it is not all the time. There are cases where order does not matter. That is why I try to keep same order all the time. Please see why-dsolve-gives-different-answer-depending-on-order-of-variables-ux-t-vs where I said at the top and ordering the variables the same elsewhere – Nasser Mar 19 '20 at 5:03
• There was also a bug in NDSolve posted recently where the order made mattered mathematica.stackexchange.com/questions/215945/… – Nasser Mar 19 '20 at 5:07