@JacquesC: Thank you for your interesting comments. @Tim Van Dusen: Thank you VERY MUCH. Mystery solved, that is exactly why I had problems when using loops but not in the other case. ;-) And I thought I had triple-checked for small mistakes...

Hi again, The worksheet is available here. I trimmed things a bit, so forget about the previously mentioned N. Firstly, it calculates Izr[0,0], Izr[0,1] and Izr[1,1] using loops; and then, it calculates (same equations and constants) Izr00, Izr01 and Izr11. One can see that the first two terms using both methods have the same values. However Izr[1,1] and Izr11 are different, the former displays unevaluated integrals while the latter show a neat result.

@Tim Van Dusen: Thanks for your reply, but that is not my problem. My problem is that when I use the loops the integrals are not evaluated, i.e. calculated providing me with a number. However, if I do not use a loop, they are calculated correctly. @Mariner: Thanks for your reply, as soon as I can I will post it. By the way, I am using Maple 11.