Hello Jean,
there's no mystic, Gamma() in SMath Studio is an approximated function, defined for not-null positive values. In your X(4) you have to initialize u[1] with a small value to not hurt the first iteration in the loop.
![2017-06-12 18_21_29-SMath Studio - [Maths%20Gamma(x)%20Unsolvable.sm_].png](/en-US/file/exVChH/2017-06-12-18_21_29-SMath-Studio---_Maths_20Gamma_x__20Unsolvable_sm___png)
If you like my plugins please consider to support the program buying a license; for personal contributions to me: paypal.me/dcprojects