[Maxima] product changes float exponents

Barton Willis willisb at unk.edu
Mon Mar 31 06:09:57 CDT 2008


In rtest9, problems 67 and 68, we have

  product((x^i+1)^2.5,i,1,inf)/(x^2+1);
  ('product((x^i+1)^2.5,i,1,inf))/(x^2+1)$

  ev(taylor(%,x,0,3),keepfloat);
  1+2.5*x+3.375*x^2+6.5625*x^3$

Using 5.14.0:

  (%i1) product((x^i+1)^2.5,i,1,inf)/(x^2+1);
  `rat' replaced 2.5 by 5/2 = 2.5
  (%o1) product((x^i+1)^(5/2),i,1,inf)/(x^2+1)

  (%i2) ev(taylor(%,x,0,3),keepfloat);
  (%o2) 1+(5*x)/2+(27*x^2)/8+(105*x^3)/16+...

Isn't (%o1) wrong? I think Maxima should preserve the float exponent.
This is the way sum works:

  (%i4) sum((x^i+1)^2.5,i,1,inf)/(x^2+1);
  (%o4) sum((x^i+1)^2.5,i,1,inf)/(x^2+1)

Given that product (incorrectly) changes the exponent to 5/2, the
test ev(taylor(%,x,0,3),keepfloat) should fail. But due to the way the
test suite checks taylor polynomials, it doesn't :(

Barton


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