Opened 12 years ago

Closed 12 years ago

Last modified 12 years ago

#1846 closed defect (invalid)

maxima looses minus signs in symbolic expression

Reported by: jkantor Owned by: was
Priority: major Milestone: sage-duplicate/invalid/wontfix
Component: calculus Keywords:
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Description

Consider

log(a*e^(-a*x-b)).simplify_exp()

This gets expanded correctly, however

log(a*e^(-a*x-b)/(1+exp(-a*x-b))^2 ).simplify_exp()

appears to lose the minus signs on ax and b.

Change History (3)

comment:1 Changed 12 years ago by was

  • Resolution set to invalid
  • Status changed from new to closed

Actually Sage is right and you are wrong, because of the identity log(e^(-a*x - b) + 1) == log(e^(a*x + b) + 1) - a*x - b which one can derive from Taylor expansions of log(x+1) and log(x^(-1) + 1). See below.

h = log(exp(-a*x-b) + 1)
h == h.simplify_exp()
///
log(e^(-a*x - b) + 1) == log(e^(a*x + b) + 1) - a*x - b
log(x+1).taylor(x,0,5)
///
x - x^2/2 + x^3/3 - x^4/4 + x^5/5
log(x^(-1)+1).taylor(x,0,5)
///
-log(x) + x - x^2/2 + x^3/3 - x^4/4 + x^5/5

comment:2 Changed 12 years ago by was

  • Milestone set to sage-2.10.1

comment:3 Changed 12 years ago by mabshoff

  • Milestone changed from sage-2.10.1 to sage-duplicate/invalid
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