79 (seventy-nine) is the natural number following 78 and preceding 80.
In mathematics 79 is a Fortunate prime, a circular prime,Numbers such that every cyclic permutation is a prime. a happy prime, a Higgs prime, a lucky prime, a regular prime, a cousin prime with 83, and a sexy prime with 73. Since it is a prime number of the form , it is a Gaussian prime. It is a Pillai prime because 23! + 1 is divisible by 79, but 79 is not one more than a multiple of 23. It is a right-truncatable prime, because when the last digit (9) is removed, the remaining number (7) is still prime. Because 97 is prime, it is an emirp and a permutable prime in base 10.
79 is the smallest prime number p for which the real quadratic field Q[] has class number greater than 1 (namely 3).H. Cohen, A Course in Computational Algebraic Number Theory, GTM 138, Springer Verlag (1993), Appendix B2, p.507. The table lists fields by discriminant, which is 4p for Q[] when p is congruent to 3 modulo 4, as is the case for 79, so the entry appears at discriminant 316.
79 is the smallest number that cannot be represented as a sum of fewer than 19 fourth powers.
Similarly to how the decimal expansion of 1/89 gives Fibonacci numbers, 1/79 gives Pell numbers, that is:
79 is a Leyland prime of the second kind. using 2 & 7: 79 = .
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