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Question: Strange results of a numerical integration

Question:Strange results of a numerical integration

comte_wang 4 Maple
February 11 2006
0
I try to calculate a numberical integration in 2 layer for loops(outer 21 times, inner 128 times): evalf(int(erfc(sqrt(B*xgms1*x/(x*xgmi1+xgmn1)))/gamav*exp(-1*x/gamav),x=0..infinity)), i.e., the kenerl is erfc(ax/(cx+d))×exp(-ax). But when "gamav" > 100000, the calculation time is too long to bear. So I try to do some approximation and change the upper bound into 10*gamav, 30*gamav, 50*gamav and 100*gamav. But I got the following results: BER1a1[18]=0.0001004773804, BER1a2[18]=0.0000365034983, BER1a3[18]=0.0000000096250, BER1a4[18]=0.0000000000001. The trend of results is bigger the upper bound smaller the results. Obviously, the results is wrong. When "gamav" is a smaller number, such as 1000, the four integration results is the same as when the upper bound is "infinity". Why? How can I do approximate for the integration? Thank you!
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