How to make algebraic substitutions?

Question

I would like to simplify complicated expressions by defining some variable that is a combination of other variables that appear in the expression, but without eliminating the original variables.

As a simple example, I would like to convert

a^2 + a*b^2 + b

to

a*c + b

by introducing the relation

c == a + b^2

I saw this question, but the answers require that one variable be eliminated. The best expressions I could get with those strategies are

b - b^2 c + c^2
a*c - Sqrt[-a + c]
a*c + Sqrt[-a + c]

depending on whether I eliminate a or b, but these are not meaningfully simpler than the original expression. Is there any way to introduce a new variable without eliminating one of the dependent variables in the expression? If it matters, I have Mathematica 10.3.

Check 'PolynomialReduce'. – Daniel Lichtblau Dec 24 '15 at 15:08 — Daniel Lichtblau, Dec 24 '15 at 15:08

score 7 · Accepted Answer · answered Dec 23 '15 at 17:12

7

Simplify[a^2 + a*b^2 + b, b^2 == c - a]

(*  b + a c  *)

Or

a^2 + a*b^2 + b /. b^2 -> c - a // Simplify

(*  b + a c  *)

answered Dec 23 '15 at 17:12

Bob Hanlon

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score 4 · Answer 2 · answered Dec 23 '15 at 17:33

4

Also

Simplify[a^2 + a*b^2 + b, Assumptions -> {c == a + b^2}]

works.

answered Dec 23 '15 at 17:33

m_goldberg

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1

In Simplify or FullSimplify it is not necessary to to use the Assumptions -> before the assumptions. – Bob Hanlon Dec 23 '15 at 17:43

How to make algebraic substitutions?

2 Answers2