Wright, E.M and Hardy, G.H. An Introduction to the Theory of Numbers.

Oxford: Oxford University Press, 1938.

Chapter five, “Congruences and Residues,” provides an explanation and summary of Euler’s fuction, the mathematical basis for programming the code of Agrippa.

“5.5. Euler’s Function…any set of phi(mu) residues, one from each class, is called a complete set of residues prime to mu.” (Facsimile image of excerpted text available in Documents.)