Havil, J. (2003). *Gamma : exploring Euler’s constant*. Princeton University Press. https://ia801407.us.archive.org/15/items/X8QX87XSQY7QX7XSQXS98XQX98/%5BJulian_Havil%5D_Gamma_Exploring_Euler%27s_Constant%28BookZZ.org%29.pdf