If a is an arbitrary integer relatively prime to n and g is a primitive root of n, then there
exists among the numbers 0, 1, 2, ..., exactly one number
such that

The number is then called the discrete logarithm (or generalized multiplicative order; Schneier 1996,
p. 501) of a with respect to the base g modulo n. Note that Nagell (1951, p. 112) instead uses the term "index"
and writes

For example, the number 7 is the least positive primitive root of n = 41, and since
,
Nagell, T. "Exponent of an Integer Modulo n" and "The Index Calculus." §31 and 33 in Introduction to Number Theory. New York: Wiley, pp. 102-106 and 111-115, 1951.
Schneier, B Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd ed. New York: Wiley, 1996.

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