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 A340487 a(n) is the least prime that is the concatenation of two n-digit primes, or 0 if there are none. 2
 23, 1117, 101107, 10091021, 1000710181, 100003100129, 10000031000171, 1000001910000349, 100000007100000541, 10000000071000000349, 1000000001910000000319, 100000000003100000000063, 10000000000391000000000903, 1000000000003710000000000259, 100000000000031100000000000403 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: a(n) > 0 and the first n digits of a(n) = A003617(n). - Chai Wah Wu, Jan 13 2021 LINKS Chai Wah Wu, Table of n, a(n) for n = 1..500 (n = 1..380 from Robert Israel) EXAMPLE a(3) = 101107 is prime and the concatenation of the two 3-digit primes 101 and 107. 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) then return 10^d*a+b fi;   od od:   FAIL end proc: f(1):= 23: map(f, [\$1..20]); CROSSREFS Cf. A003617. Sequence in context: A126741 A273477 A202656 * A321569 A061063 A100768 Adjacent sequences:  A340484 A340485 A340486 * A340488 A340489 A340490 KEYWORD nonn,base AUTHOR J. M. Bergot and Robert Israel, Jan 10 2021 STATUS approved

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Last modified June 16 10:30 EDT 2021. Contains 345056 sequences. (Running on oeis4.)