This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A066811 #23 Aug 28 2025 19:39:43
%S A066811 3,19,31,67,97,5139
%N A066811 Numbers k such that the concatenation of odd numbers from 1 to k is a prime.
%C A066811 a(7) > 50000. - _Michael S. Branicky_, Aug 28 2025
%F A066811 a(n) = 2*A046036(n) - 1. - _Michel Marcus_, Jan 31 2014
%e A066811 19 is a term because 135791113151719 is a prime.
%t A066811 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 A066811 (Python)
%o A066811 from sympy import isprime
%o A066811 def agen():
%o A066811 k, str1tok = 1, '1'
%o A066811 while True:
%o A066811 if isprime(int(str1tok)): yield k
%o A066811 k, str1tok = k + 2, str1tok + str(k + 2)
%o A066811 g = agen()
%o A066811 print([next(g) for i in range(5)]) # _Michael S. Branicky_, Mar 19 2021
%Y A066811 Cf. A000040, A005408, A046036, A048847.
%K A066811 nonn,base,more,changed
%O A066811 1,1
%A A066811 _Patrick De Geest_, Jan 20 2002
%E A066811 a(6) from _Ryan Propper_, Aug 26 2005