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A340539 a(n) is the least prime that is the concatenation of two n-digit primes, and such that the concatenation of the same primes in the other order is also prime or 0 if no such prime exists. 1
37, 1123, 101107, 10091789, 1000710709, 100003100363, 10000031000303, 1000001910002521, 100000007100010321, 10000000071000000349, 1000000001910000000799, 100000000003100000009939, 10000000000391000000012387, 1000000000003710000000034573, 100000000000031100000000014113 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: a(n) > 0 and for n > 1 the first n digits of a(n) = A003617(n). - Chai Wah Wu, Jan 13 2021

LINKS

Robert Israel, Table of n, a(n) for n = 1..171

EXAMPLE

For n=4, 1009, 1789, 10091789 and 17891009 are all prime.

MAPLE

f:= proc(d) local P, a, b;

  a:= prevprime(10^(d-1));

  do

    a:= nextprime(a);

    if a > 10^d then return FAIL fi;

    b:= prevprime(10^(d-1));

    do

      b:= nextprime(b);

      if b > 10^d then break fi;

      if isprime(10^d*a+b) and isprime(10^d*b+a) then return 10^d*a+b fi;

  od od:

  FAIL

end proc:

f(1):= 37:

map(f, [$1..20]);

CROSSREFS

Cf. A003617, A340487.

Sequence in context: A253245 A168166 A168165 * A219309 A099201 A218841

Adjacent sequences:  A340536 A340537 A340538 * A340540 A340541 A340542

KEYWORD

nonn,base

AUTHOR

J. M. Bergot and Robert Israel, Jan 10 2021

STATUS

approved

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Last modified January 28 04:30 EST 2022. Contains 350654 sequences. (Running on oeis4.)