# How to make algebraic substitutions?

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

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  *)


Also

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


works.

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