§14.32 in Titchmarsh, E. C. (1986). The theory of the Riemann zeta-function (Second ed.). New York: The Clarendon Press, Oxford University Press. ISBN0-19-853369-1. MR0882550.
Gårding, Lars (1997). Some points of analysis and their history. University Lecture Series. Vol. 11. Providence, RI: American Mathematical Society. pp. 31–35. ISBN0-8218-0757-9. MR1469493.