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%I #19 Mar 19 2021 10:42:48
%S 3,19,31,67,97,5139
%N Numbers k such that the concatenation of odd numbers from 1 to k is a prime.
%F a(n) = 2*A046036(n) - 1. - _Michel Marcus_, Jan 31 2014
%e 19 is a term because 135791113151719 is a prime.
%t p = ""; Do[p = p <> ToString[2*n+1]; If[PrimeQ[ToExpression[p]], Print[2*n+1]], {n, 0, 2569}] (* _Ryan Propper_, Aug 26 2005 *)
%o (Python)
%o from sympy import isprime
%o def agen():
%o k, str1tok = 1, '1'
%o while True:
%o if isprime(int(str1tok)): yield k
%o k, str1tok = k + 2, str1tok + str(k + 2)
%o g = agen()
%o print([next(g) for i in range(5)]) # _Michael S. Branicky_, Mar 19 2021
%Y Cf. A000040, A005408, A048847.
%K base,nonn
%O 1,1
%A _Patrick De Geest_, Jan 20 2002
%E a(6) from _Ryan Propper_, Aug 26 2005