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 A125569 #15 Jun 30 2024 06:26:10
%S A125569 3,10,11,38,735,1083,2063
%N A125569 Numbers k such that Sum_{j=1..k-1} j*5^(j-1) is prime.
%C A125569 No more terms through 10^4.
%C A125569 No more terms through 5*10^4. - _Michael S. Branicky_, Jun 30 2024
%t A125569 s = 0; Do[s += (x-1)*5^(x-1)/5; If[PrimeQ[s], Print[x]], {x, 10^4}]
%t A125569 Select[Range[2100],PrimeQ[Sum[x*5^(x-1),{x,#-1}]]&] (* _Harvey P. Dale_, Feb 09 2023 *)
%Y A125569 Cf. A119529.
%K A125569 hard,more,nonn
%O A125569 1,1
%A A125569 _Ryan Propper_, Jan 01 2007
%E A125569 Name simplified by _Jon E. Schoenfield_, Sep 23 2018