A238499 Primes which are the concatenation of two primes in exactly three ways.
3137, 3797, 13997, 19937, 19997, 23911, 23929, 29173, 29311, 31193, 37337, 37397, 43397, 59929, 73331, 78737, 79337, 103997, 109397, 127997, 139967, 173347, 173359, 193337, 193373, 193877, 199337, 199373, 199967, 229373, 233113, 233329, 233353, 233617
Offset: 1
Examples
13997 is in the sequence because (13, 997), (139, 97), (1399, 7) are all primes, so there are three ways.
Links
- Giovanni Resta, Table of n, a(n) for n = 1..10000
Programs
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Mathematica
spl[n_] := Block[{d = IntegerDigits@n, c = 0, z}, z = Length@d; Do[If[PrimeQ@ FromDigits@ Take[d, k] && d[[k + 1]] > 0 && PrimeQ@ FromDigits@ Take[d, k - z], c++], {k, z - 1}]; c]; Select[ Prime@ Range@ 20000, spl[#] == 3 &] (* Giovanni Resta, Mar 03 2014 *)
Extensions
Example clarified by Harvey P. Dale, Jun 09 2025