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 and )
- An isolated prime without a twin prime, as 77 and 81 are composite.
- The smallest prime number p for which the real quadratic field Q[] has class number greater than 1 (namely 3).
- A cousin prime with 83.
- An emirp in base 10, because the reverse of 79, 97, is also a prime.
- A Fortunate prime.
- A circular prime.
- A prime number that is also a Gaussian prime (since it is of the form ).
- A happy prime.
- A Higgs prime.
- A lucky prime.
- A permutable prime, with ninety-seven.
- A Pillai prime, because 23! + 1 is divisible by 79, but 79 is not one more than a multiple of 23.
- A regular prime.
- 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 and Leyland prime of the second kind, using 2 & 7 (<math>2^7-7^2</math>)
thumb|Signage for table 79 at a restaurant