I have a kind of dumb question I'm afraid.

Given two complicated expressions, is there a fast check to see whether one is a rational multiple of the other. I mean given expressions expr1, expr2 something like NumberQ[expr1/expr2]. However NumberQ[Simplify[expr1/expr2]] does exactly what I need but might take forever! Is there a faster way?

By the way I have no idea how to tag this question...

  • $\begingroup$ If Simplify is too slow I think you need to do it approximately like Rationalize[expr1/expr2, 10^(-tol)] === Rationalize[expr1/expr2, 10^(-2*tol)] with e.g. tol = 100 $\endgroup$
    – Coolwater
    Sep 30 '16 at 9:48
  • $\begingroup$ Could you add a "typical" example of expr1 and expr2? $\endgroup$
    – jkuczm
    Sep 30 '16 at 10:20
  • $\begingroup$ Both expressions are polynomials with a lot of terms adding up in n variables with integer coefficients here. They are found themselves from certain types of graphs which I input. Sometimes two completely different graphs yield proportional polynomials although you can only see it after a lot of algebraic simplifications $\endgroup$
    – Hamed
    Sep 30 '16 at 14:34

My approach is to evaluate the polynomials with random integers assigned to the variables. This is likely to be very fast.

For test purposes, let us define 3 polynomials in 7 variables

expr1a = (-2 + a[1]) (-5 + a[2]) (5 + a[3]) (4 + a[4]) (1 + a[5]) a[6] (-4 + a[7]);
expr1b = Expand[13/7 expr1a];
expr2 = expr1b + 1;

Define functions

randominstance[x_] := Block[{a},
  Do[a[i] = RandomInteger[{-1000, 1000}], {i, 7}];
ratiocheck[x1_, x2_] := SameQ[randominstance[x1/x2], randominstance[x1/x2]]

Test that the functions give the expected results on our test polynomials

ratiocheck[expr1a, expr1b]
(* True *)

ratiocheck[expr1a, expr2]
(* False *)

There is a risk of concluding that two polynomials have a constant ratio, when in fact they do not, but you can do multiple checks.

  • $\begingroup$ Thanks this works beautifully... I guess a triple check is quite safe... $\endgroup$
    – Hamed
    Sep 30 '16 at 22:27

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