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I have an expression: $p=a\;b\; x + b^2\; y + a\;c\; z$. I want to substitute $a\;b=1$, $b^2 = 2$ and $a\;c = 4$ to obtain $p = x + 2y + 4z$.
How can I tell Mathematica to do that? I dont know how to start.

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1 Answer 1

up vote 8 down vote accepted

A reliable approach would use the third argument of Reduce as variables to eliminate (see Behavior of Reduce with variables as domain)

Reduce[{p == a b x + b^2 y + a c z, a b == 1, b^2 == 2, a c == 4}, {p}, {a, b, c}]
p == x + 2 y + 4 z

In the former editions of Mathematica (ver <= 4) Reduce used the third argument for eliminating another variables. It can be still used this way though it is not documented anymore, however its trace could be found in SystemOptions["ReduceOptions"].
If Reduce didn't work this way one would exploit Solve (it is still supposed to eliminate variables), e.g.:

Apply[ Equal, Solve[{p == a b x + b^2 y + a c z, a b == 1, b^2 == 2, a c == 4}, 
       {p}, {a, b, c}], {2}][[1, 1]]

or Eliminate:

Eliminate[{p == a b x + b^2 y + a c z, a b == 1, b^2 == 2, a c == 4}, 
          {a, b, c}] // Reverse

or even simply appropriate rules replacement (in general this approach cannot be used seamlessly though)

p == a b x + b^2 y + a c z /. {a b -> 1, b^2 -> 2, a c -> 4} // TraditionalForm

enter image description here

Edit

It would be reasonable to mention another two functions useful in similar tasks. Taking this polynomial identiclly equal to zero:

poly = p - a b x - b^2 y - a c z;

we can rewrite it in terms of another three polynomials which are also identically zeros by the assumptions:

{poly1, poly2, poly3} = {a b - 1, b^2 - 2, a c - 4};

thus we know that the resulting polynomial will be equal to zero as well:

Last @ PolynomialReduce[ poly, {poly1, poly2, poly3}, {a, b, c, p, x, y, z}] == 0
p - x - 2 y - 4 z == 0

similarily we can find a Groebner basis of polynomials { poly, poly1, poly2, poly3} eliminating unwanted variables {a, b, c}:

First @ GroebnerBasis[{ poly, poly1, poly2, poly3}, {x, y, z}, {a, b, c},
                       MonomialOrder -> EliminationOrder] == 0

These two methods are more useful when we want to find different representations of (polynomial) expressions in polynomial rings, thus we needn't assume that polynomials { poly, poly1, poly2, poly3} identically vanish.

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Thank you for your answer. –  minthao_2011 Feb 11 at 15:50
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