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 A320442 Primes which are the concatenation of primes p and 2*p-1. 2
 23, 367733, 6611321, 6911381, 9971993, 14592917, 20114021, 25575113, 27075413, 30616121, 39677933, 40278053, 41778353, 43578713, 46399277, 574911497, 621112421, 622912457, 696113921, 753715073, 756115121, 795115901, 819116381, 862917257, 868917377, 961919237, 1078921577, 1083721673, 1131122621 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms == 5 (mod 6). LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 367733 is in the sequence because 367, 2*367-1=733, and 367733 are all primes. MAPLE f:= proc(p) local q, r;    if not isprime(p) then return NULL fi;    q:= 2*p-1;    r:= p*10^(1+ilog10(q))+q;    if isprime(r) and isprime(q) then r else NULL fi; end proc: map(f, [2, seq(i, i=7..20000, 6)]); PROG (PARI) forprime(p=1, 1e4, my(cc=eval(concat(Str(p), Str(2*p-1)))); if(ispseudoprime(2*p-1) && ispseudoprime(cc), print1(cc, ", "))) \\ Felix FrÃ¶hlich, Jan 09 2019 CROSSREFS Cf. A005382, A168165. Sequence in context: A138763 A156176 A013772 * A034247 A050234 A124993 Adjacent sequences:  A320439 A320440 A320441 * A320443 A320444 A320445 KEYWORD nonn,base AUTHOR J. M. Bergot and Robert Israel, Jan 09 2019 STATUS approved

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Last modified February 28 04:39 EST 2020. Contains 332321 sequences. (Running on oeis4.)