

A068780


Composite numbers n such that n+1 is also composite.


13



8, 9, 14, 15, 20, 21, 24, 25, 26, 27, 32, 33, 34, 35, 38, 39, 44, 45, 48, 49, 50, 51, 54, 55, 56, 57, 62, 63, 64, 65, 68, 69, 74, 75, 76, 77, 80, 81, 84, 85, 86, 87, 90, 91, 92, 93, 94, 95, 98, 99, 104, 105, 110, 111, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124
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OFFSET

1,1


COMMENTS

For all primes p, neither p nor p1 is in the sequence.  Jon Perry, Oct 12 2014


LINKS



FORMULA

There are x  2x/log x + O(x/log^2 x) members up to x. The coefficient of the next asymptotic term depends on the quantitative version of the twin prime conjecture (though it can be bounded between 0.6796763684 and 2.4885722184, with the former conjectured to be the case).  Charles R Greathouse IV, Dec 19 2018


MAPLE

q:= n> andmap(not isprime, [n, n+1]):


MATHEMATICA

Select[ Range[2, 200], !PrimeQ[ # ] && !PrimeQ[ # + 1] &]


PROG

(Magma) [n: n in [1..200]  not IsPrime(n) and not IsPrime(n+1)]; // Vincenzo Librandi, Oct 17 2014


CROSSREFS



KEYWORD

nonn,easy


AUTHOR



EXTENSIONS



STATUS

approved



