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Problems with Assumptions for Integrate and Reduce

Posted 12 years ago

Hi - Im using Mathematica 9 Home edition and having some trouble. This is kind of a messy equation, but maybe there is something simple I am missing. The problem came about with the following equation. All parameters are positive, and I know that (1 + [Alpha]) (1 - [Alpha] (t/[Lambda])^k) must be positive for everything to be "nice", but I don't get what Mathematica is doing here:

In[87]:= Assuming[x > 0 && ? > 0 && k > 0 && ? > 0, 
 FullSimplify[
  Integrate[(
   k (1 + ?) (1 - ? (t/?)^k)^(
    1/?) (t/?)^k)/t, {t, 0, x}]]]

Out[87]= ConditionalExpression[?^-k (?^k + 
    E^((I ?)/?) (-1 + ? (x/?)^k)^(
     1/?) (x^k ? - ?^k)), (?/x)^
   k < ? && x ?^(1/k) <= ?]

Which I know is wrong, there should be no E^((I [Pi])/[Alpha]) in there. Also, the two constraints at the answer are mutually contradictory, they can never evaluate to true. Furthermore, if I set [Lambda]==1 in the assumptions, the E^((I [Pi])/[Alpha]) disappears and I get a correct answer. I think it might have to do with this:

Assuming[x > 0 && ? > 0 && k > 0 && ? > 0, Reduce[x ?^(1/k) <= ?, ?]]

During evaluation of In[101]:= Reduce::nsmet: This system cannot be solved with the methods available to Reduce. >>

I don't see what the problem is - it should be easily reduced, all parameters are positive.

Thanks for any help -

POSTED BY: Paul Reiser
6 Replies
POSTED BY: Sean Clarke
Posted 11 years ago

I have tried to narrow things down. The E^((I [Pi])/\a) term does not make things complex, necessarily. My problem boils down to

Assuming[k > 0 && a > 0 && y > 1/a, FullSimplify[Exp[I [Pi]/a] (-1 + a y)^(1/a) == (1 - a y)^(1/a)]]

yields True, but when I substitute y^k for y in the above expression, it returns unsimplifed. I think that for k>0 there should be no problem. Do I have the mathematics wrong, or is Mathematica dropping the ball here?

Thanks Paul Reiser

POSTED BY: Paul Reiser

Does seem to be a strange result. Will take a look.

POSTED BY: Daniel Lichtblau
Posted 11 years ago
POSTED BY: Paul Reiser

There seems to be an issue again with how your code is being posted on the forum. If you can see, there's square brackets around Alpha and Lambda and some of it is a hyperlink.

So we know exactly what you're seeing, can you copy the code, run it as plaintext, and then code as plaintext the output?

What version of Mathematica are you using?

POSTED BY: Sean Clarke

In[2]:= FullSimplify[ Integrate(k (1 + [Alpha]) (1 - [Alpha^ k)^(1/[Alpha]) (t/[Lambda])^k)/t, {t, 0, x}]]

Out[2]= ConditionalExpression[ 1 - (1 - [Alpha] (x/[Lambda])^k)^( 1/[Alpha]) + [Alpha] (1 - [Alpha] (x/[Lambda])^k)^( 1/[Alpha]) (x/[Lambda])^k, Re[k] > 0]

POSTED BY: S M Blinder
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