[Help-gsl] sign in gsl_sf_legendre_Plm / gsl_sf_legendre_sphPlm

[Denis Davydov]

Dear all, 

I am trying to use the associated Legendre polynomials, 
but I am confused about the sign. 
I would expect P_1^1(cos(phi)) = sin(phi) > 0 and
therefore to have the normalised polynomial also positive. 
However, it seems that there is a (-1)^m multiplier 
compared to the expression I see in several books. 
Could someone please clarify this issue for me?

Kind regards,
Denis