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.
37, 1123, 101107, 10091789, 1000710709, 100003100363, 10000031000303, 1000001910002521, 100000007100010321, 10000000071000000349, 1000000001910000000799, 100000000003100000009939, 10000000000391000000012387, 1000000000003710000000034573, 100000000000031100000000014113
Offset: 1
Examples
For n=4, 1009, 1789, 10091789 and 17891009 are all prime.
Links
- Robert Israel, Table of n, a(n) for n = 1..171
Programs
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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]);
Comments