# Complex numbers with thrust::complex

Hello!

I have a Monte Carlo integrator with quite large complex integrand(I consider only real part at the first stage). Integrand itself contains the product of double-valued and thrust::complex-valued functions. The result is absolutely unpredictable(for me at least):
For example:

`thrust::complex<double> part11, part12;`
`double part11r, part12r;`

`part11r=(-(FDEl_k2P*FDHo_dsqrt_mo_kqP_p) + FDEl_dsqrt_mo_k2pqP_m*(-1.0 + FDEl_k2P + FDHo_dsqrt_mo_kqP_p))*VPG_qP; // all functions and variables are double-valued`
`part11=-DWEn(k,omegaIn,"c","c","c","v",k,k2P,dsqrt_mo_k2pqP_m,dsqrt_mo_kqP_p)*part11r; //DWEn returns thrust::complex<double>.`

`part12r=(-((-1 + FDEl_dsqrt_mo_kqP_m)*FDHo_k2P) + (FDEl_dsqrt_mo_kqP_m - FDHo_k2P)*FDHo(dsqrt_mo_k2pqP_m))*VPG_qP;`
`part12=DWEn(k,omegaIn,"c","v","v","v",dsqrt_mo_kqP_m,k2P,dsqrt_mo_k2pqP_m,k)*part12r;`

`thrust::complex<double> integrand;`
`integrand=part11+part12;`
`return integrand.real();`
This works fine(can be calculated by MC at least and does not return NaN)

But without some local variables:

`integrand =-DWEn(k,omegaIn,"c","c","c","v",k,k2P,dsqrt_mo_k2pqP_m,dsqrt_mo_kqP_p)*(-(FDEl_k2P*FDHo_dsqrt_mo_kqP_p) + FDEl_dsqrt_mo_k2pqP_m*(-1 + FDEl_k2P +FDHo_dsqrt_mo_kqP_p))*VPG_qP+DWEn(k,omegaIn,"c","v","v","v",dsqrt_mo_kqP_m,k2P,dsqrt_mo_k2pqP_m,k)*(-((-1 + FDEl_dsqrt_mo_kqP_m)*FDHo_k2P) + (FDEl_dsqrt_mo_kqP_m - FDHo_k2P)*FDHo(dsqrt_mo_k2pqP_m))*VPG_qP;`

The integration returns NaN.

I can present other scenarios of strange behavior(the answer will be nan or not depends on the introducing of local variables). I do not understand a logic of how to work with huge product of real(double) and comlex(thrust::complex) valued functions.

I will be infinitely grateful for the help.

The solution was suggested by Mat Colgrove. The problem was in used resources(the integrand is quite huge): kernel is failing due to too many resources being used. The changing of number of threads from 1024 to 512 fixed the appearing of nan result.

Mathew, thank you very much.