A275319 Numbers n such that n concatenated with n+1 is not a prime.

1, 3, 4, 5, 7, 9, 10, 11, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 49, 51, 52, 53, 54, 55, 57, 58, 59, 60, 61, 63, 64, 65, 66, 67, 69, 70, 71, 72, 73, 74, 75, 76

Offset: 1

Author

Vincenzo Librandi, Jul 23 2016

Keywords

• nonn
• easy
• base

Comments

Complement of A030457.

Conjecture: a(n) ~ n. - Charles R Greathouse IV, Jul 23 2016

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A030457.

Programs

Magma

[n: n in [1..500] | not IsPrime(Seqint(Intseq(n+1) cat Intseq(n)))]

Mathematica

Select[Range[100], !PrimeQ[FromDigits[Join[IntegerDigits[#], IntegerDigits[# + 1]]]] &]

PARI

is(n)=!isprime(eval(Str(n,n+1))) \\ Charles R Greathouse IV, Jul 23 2016