"double complex csqrt(double complex "z");"
"float complex csqrt(float complex "z");"
"long double complex csqrt(long double complex "z");"
Calculate the square root of a given complex number, with nonnegative real part, and with a branch cut along the negative real axis. (That means that csqrt(-1+eps*I) will be close to I while csqrt(-1-eps*I) will be close to -I, if eps is a small positive real number.)