Hi, log(t)/2=-sum(((cosh(x)-sqrt(cosh(x)^2+t-1))^n*cosh(n*x))/n,n=1..infinity); log(t)/2=-sum(((cos(x)-sqrt(cos(x)^2+t-1))^n*cos(n*x))/n,n=1..infinity); The exceptions for the values of x are obvious for my previous message. Best regards