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A154966
Primes p such that the concatenation of p and prime(p) is composite.
1
3, 5, 7, 11, 13, 19, 29, 31, 37, 43, 47, 53, 59, 67, 73, 79, 89, 97, 101, 103, 107, 109, 113, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317
OFFSET
1,1
LINKS
EXAMPLE
The concatenation of prime 3 and the third prime, 5, is the composite 35, hence 3 is in the sequence.
The concatenation of prime 29 and the 29th prime, 109, is the composite 29109 = 3*31*313, hence 29 is in the sequence.
MATHEMATICA
Select[Prime[Range[80]], CompositeQ[FromDigits[Flatten[IntegerDigits[ {#, Prime[ #]}]]]]&] (* Harvey P. Dale, Jan 07 2016 *)
PROG
(Magma) [ p: p in PrimesUpTo(320) | not IsPrime(StringToInteger(IntegerToString(p) cat IntegerToString(NthPrime(p)))) ];
CROSSREFS
Cf. A000040 (primes), A002808 (composites), A045532.
Sequence in context: A055072 A059334 A130761 * A072667 A092729 A290817
KEYWORD
nonn,base,easy,less
AUTHOR
EXTENSIONS
Edited and extended beyond a(6) by Klaus Brockhaus, Jan 20 2009
STATUS
approved