# Why does Simplify not reduce the expression?

I have a long expression and it includes some terms with long polynomial coefficients.

For example,

expression =
Log[a] (48 A ((2 - 4 a + 3 a^2) eA F^2 + A eF ((2 - 4 a + a^2) A - 4 (-1 + a)^2 F)) - (-1 + a) A^2 Log[a/(-1 + a)]^2 (A (-96 + 192 a + 420 a^3 R - 779 a^4 R
+ 657 a^5 R - 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]) - 2 (-1 + a) A F Log[a/(-1 + a)] Log[(-1 + a)/b] (A (-96 + 192 a + 420 a^3 R - 779 a^4 R
+ 657 a^5 R - 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]) - (-1 + a) F^2 Log[(-1 + a)/b]^2 (A (-96 + 192 a + 420 a^3 R - 779 a^4 R + 657 a^5 R
- 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]));


where all parameters are positive with a>1. I try to Simplify it with

Simplify[expression, {Positive[R], Positive[A], Positive[F], Positive[eA], Positive[eF], Positive[b]}, Assumptions -> a > 1]


but it returns the same expression. Obviously, there are three terms multiplied by the same polynomial (A (-96 + 192 a + 420 a^3 R - 779 a^4 R + 657 a^5 R - 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]). But I don't know how to tell Mathematica to understand this.

I found with the first term in the parentheses absent, Mathematica knows how to do this. But in my real problem, I have a long expression with many instances like the abovementioned. Thank you very much!

expression2 = Log[a] ((*48 A ((2 - 4 a + 3 a^2) eA F^2 + A eF ((2 - 4 a + a^2) A - 4 (-1 + a)^2 F))*) - (-1 + a) A^2 Log[a/(-1 + a)]^2 (A (-96 + 192 a + 420 a^3 R - 779 a^4 R
+ 657 a^5 R - 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]) - 2 (-1 + a) A F Log[a/(-1 + a)] Log[(-1 + a)/b] (A (-96 + 192 a + 420 a^3 R - 779 a^4 R
+ 657 a^5 R - 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]) - (-1 + a) F^2 Log[(-1 + a)/b]^2 (A (-96 + 192 a + 420 a^3 R - 779 a^4 R + 657 a^5 R
- 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b]));

Simplify[expression2, {Positive[R], Positive[A], Positive[F], Positive[eA], Positive[eF], Positive[b]}, Assumptions -> a > 1]

(*-(-1 + a) Log[a] (A Log[a/(-1 + a)] + F Log[(-1 + a)/b])^2 (A (-96 + 192 a + 420 a^3 R - 779 a^4 R + 657 a^5 R - 243 a^6 R + 29 a^7 R - 12 a^2 (4 + 7 R)) + 12 (-1 + a)^3 a^2 (7 - 14 a + 3 a^2) F R Log[b])*)


FullSimplify[expression, {Positive[R], Positive[A], Positive[F],

• Thank you. I actually tried FullSimplify but it runs for a very long time before aborting manually. Is there any other method? Commented Feb 11, 2022 at 12:02