[math-fun] Bale problem's

Hello In an unfortunate attempt to solve zeta (2n + 1), I became interested in certain functions. I was able to find: cos (x / sqrt (2)) * cosh (x / sqrt (2)) = sum ((- 1) ^ n * x ^ (4 * n) / (4 * n)!,n=0..infinity) but I am lost to the next sum. sum ((- 1) ^ n * x ^ (3 * n) / (3 * n)!,n=0..infinity) = ??? Do you know the function returned by this sum? I just noticed that the solutions of the equation sum ((- 1) ^ n * x ^ (3 * n) / (3 * n)!,n=0..infinity) = 0 are substantially close to: (2 * n + 1) * Pi / sqrt (3) Is there more information on this?