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A155030 Primes p=A000040(n) with prime index n such that the concatenation n//p is a composite number. 1
5, 11, 17, 31, 41, 67, 109, 127, 157, 191, 211, 241, 277, 331, 367, 401, 461, 509, 547, 563, 587, 599, 617, 739, 773, 797, 859, 877, 919, 967, 991, 1031, 1063, 1087, 1153, 1171, 1201, 1217, 1297, 1409, 1447, 1471, 1499, 1523, 1597, 1621, 1669, 1723, 1741 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Demanding that the index is not a prime would lead to A155025 instead.

LINKS

Table of n, a(n) for n=1..49.

EXAMPLE

For the index n=3, a prime, p = prime(3)=5, the concatenation 35 is composite, so p=5 is added to the sequence.

For the index n=5, a prime, p = prime(5)=11, the concatenation 511 is composite, so p=11 is added to the sequence.

For the index n=6, not a prime, nothing is added to the sequence.

For the index n=7, a prime, p = prime(7)=17, the concatenation 717 is composite, so p=17 is added to the sequence.

MATHEMATICA

Prime[#]&/@Select[Prime[Range[70]], CompositeQ[FromDigits[Join[ IntegerDigits[ #], IntegerDigits[ Prime[#]]]]]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 14 2019 *)

CROSSREFS

Cf. A000027, A000040, A002808, A155025.

Sequence in context: A088046 A155882 A087373 * A030468 A277290 A085634

Adjacent sequences: A155027 A155028 A155029 * A155031 A155032 A155033

KEYWORD

nonn,base

AUTHOR

Juri-Stepan Gerasimov, Jan 19 2009

EXTENSIONS

Definition clarified, sequence extended by R. J. Mathar, Oct 14 2009

STATUS

approved

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Last modified December 7 05:41 EST 2022. Contains 358649 sequences. (Running on oeis4.)