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A154966 Primes n such that the concatenation of n and the n-th prime 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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

Concatenation of prime 3 and third prime 5 is the composite 35, hence 3 is in the sequence.

Concatenation of prime 29 and 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).

Sequence in context: A055072 A059334 A130761 * A072667 A092729 A290817

Adjacent sequences:  A154963 A154964 A154965 * A154967 A154968 A154969

KEYWORD

nonn,base,easy,less

AUTHOR

Juri-Stepan Gerasimov, Jan 18 2009

EXTENSIONS

Edited and extended beyond a(6) by Klaus Brockhaus, Jan 20 2009

STATUS

approved

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Last modified April 23 06:08 EDT 2019. Contains 322381 sequences. (Running on oeis4.)