Euler's phi function

Euler's phi function

[′ȯi·lərz ′fī ‚fəŋk·shən]
(mathematics)
A function φ, defined on the positive integers, whose value φ(n) is the number of integers equal to or less than n and relatively prime to n. Also known as indicator; phi function; totient.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.