79 (number)

Template:Infobox number 79 (seventy-nine) is the natural number following 78 and preceding 80.

In mathematics

79 is:

An odd number.
The smallest number that can not be represented as a sum of fewer than 19 fourth powers.
The 22nd prime number (between Template:Num and Template:Num)
An isolated prime without a twin prime, as 77 and 81 are composite.<ref>Template:Cite OEIS</ref>
The smallest prime number p for which the real quadratic field Q[[[:Template:Sqrt]]] has class number greater than 1 (namely 3).<ref>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[[[:Template:Sqrt]]] when p is congruent to 3 modulo 4, as is the case for 79, so the entry appears at discriminant 316.</ref>
A cousin prime with 83.
An emirp in base 10, because the reverse of 79, 97, is also a prime.<ref>{{#invoke:citation/CS1|citation

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A circular prime.<ref> Numbers such that every cyclic permutation is a prime.</ref>
A prime number that is also a Gaussian prime (since it is of the form Template:Nowrap).
A happy prime.<ref>{{#invoke:citation/CS1|citation

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A right-truncatable prime, because when the last digit (9) is removed, the remaining number (7) is still prime.
A sexy prime (with 73).
The n value of the Wagstaff prime 201487636602438195784363.
Similarly to how the decimal expansion of 1/89 gives Fibonacci numbers, 1/79 gives Pell numbers, that is, <math>\frac{1}{79}=\sum_{n=1}^\infty{P(n)\times 10^{-(n+1)}}=0.0126582278\dots\ .</math>
A Leyland number of the second kind<ref>Template:Cite OEIS</ref> and Leyland prime of the second kind,<ref>Template:Cite OEIS</ref> using 2 & 7 (<math>2^7-7^2</math>)