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A320442
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Primes which are the concatenation of primes p and 2*p-1.
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2
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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
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OFFSET
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1,1
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COMMENTS
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All terms == 5 (mod 6).
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LINKS
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EXAMPLE
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367733 is in the sequence because 367, 2*367-1=733, and 367733 are all primes.
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MAPLE
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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)]);
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PROG
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(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 Fröhlich, Jan 09 2019
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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