79 (seventy-nine ) is the natural number following 78 and preceding 80 .
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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 73 and 83 )
An isolated prime without a twin prime , as 77 and 81 are composite.[1]
The smallest prime number p for which the real quadratic field Q [√ p ] has class number greater than 1 (namely 3).[2]
A cousin prime with 83.
An emirp in base 10 , because the reverse of 79, 97 , is also a prime.[3]
A Fortunate prime .[4]
A circular prime .[5]
A prime number that is also a Gaussian prime (since it is of the form 4n + 3 ).
A happy prime .[6]
A Higgs prime .[7]
A lucky prime .[8]
A permutable prime , with ninety-seven .
A Pillai prime ,[9] because 23! + 1 is divisible by 79, but 79 is not one more than a multiple of 23 .
A regular prime .[10]
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,
1
79
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n
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1
∞
P
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n
)
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10
−
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n
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1
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=
0.0126582278
…
.
{\displaystyle {\frac {1}{79}}=\sum _{n=1}^{\infty }{P(n)\times 10^{-(n+1)}}=0.0126582278\dots \ .}
A Leyland number of the second kind .
Signage for table 79 at a restaurant
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 [√ p ] when p is congruent to 3 modulo 4, as is the case for 79, so the entry appears at discriminant 316.