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Re: Simple integration that Mathematica can't do?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27809] Re: Simple integration that Mathematica can't do?
  • From: "Tom Hardy" <tomhardy at erols.com>
  • Date: Fri, 16 Mar 2001 04:38:10 -0500 (EST)
  • References: <98a200$cr8@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Try the replacement x=>x+1 and change the integration limits to 0 to big.  I
get (1+a)E^-a with 3.0.  Agree, Mathematica could be better. BTW is the new
version any better with regard to graphics? I never could get them to print
correctly under 3.0.


"A. E. Siegman" <siegman at stanford.edu> wrote in message
news:98a200$cr8 at smc.vnet.net...
> I'm a little surprised that Mathematica can't do the following fairly
> simple calculation:
>
>    f[x_] := Sqrt[4a^3] x Exp[-a x]
>
>    u = Integrate[ f[x] f[x - 1],  {x, 1, Infinity},
>           Assumptions -> {Re[a] > 0}]
>
> even though if I break it apart by hand into the form
>
>    u1 = 4a^3 Exp[a] Integrate[x^2  Exp[-2a x], {x, 1, Infinity},
>            Assumptions -> {Re[a] > 0}]
>
>    u2 = 4a^3 Exp[a] Integrate[x  Exp[-2a x], {x, 1, Infinity},
>            Assumptions -> {Re[a] > 0}]
>
>    u = u1 - u2
>
> it can do both of the component integrals with no problem.
>




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