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

%I #8 Sep 08 2022 08:46:17

%S 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,

%T 30,31,32,33,34,35,37,38,39,40,41,43,44,45,46,47,48,49,51,52,53,54,55,

%U 57,58,59,60,61,63,64,65,66,67,69,70,71,72,73,74,75,76

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

%C Complement of A030457.

%C Conjecture: a(n) ~ n. - _Charles R Greathouse IV_, Jul 23 2016

%H Charles R Greathouse IV, <a href="/A275319/b275319.txt">Table of n, a(n) for n = 1..10000</a>

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

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

%o (PARI) is(n)=!isprime(eval(Str(n,n+1))) \\ _Charles R Greathouse IV_, Jul 23 2016

%Y Cf. A030457.

%K nonn,easy,base

%O 1,2

%A _Vincenzo Librandi_, Jul 23 2016