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 A372262 #12 Aug 06 2024 13:37:43
%S A372262 1,1,-1,1,1,-1,1,1,-1,1,1,-1,14,2,-1,1,1,-1,1,3,-1,1,1,-1,8,1,-1,1,1,
%T A372262 -1,1,4,-1,2,1,-1,1,1,-1,483,2,-1,1,1,-1,1,2,-1,2,1,-1,1,2,-1,3,1,-1,
%U A372262 6,1,-1,1,5,-1,1,1,-1,1,1,-1,5,3,-1,1,1,-1,3,1,-1,2,4
%N A372262 a(n) = smallest number m > 0 such that n followed by m 3's yields a prime, or -1 if no such m exists.
%C A372262 a(n) = -1 when n = 3*k because no matter how many 3's are appended to n, the resulting number is always divisible by 3 and therefore cannot be prime.
%C A372262 a(n) = -1 when n = 37037*k + 2808, 3666, 4070, 9287, 18799, 21574, 28083, 30558, 33300, 33740, 36663 or 36707, because n followed by any positive number, m say, of 3's is divisible by at least one of the primes {7,11,13,37}.
%C A372262 a(817) > 300000 or a(817) = -1.
%H A372262 Toshitaka Suzuki, <a href="/A372262/b372262.txt">Table of n, a(n) for n = 1..816</a>
%e A372262 a(20)=3 because 203 and 2033 are composite but 20333 is prime.
%t A372262 snm[n_]:=Module[{k=1},If[Mod[n,3]==0,-1,While[CompositeQ[FromDigits[ PadRight[ IntegerDigits[ n],k+ IntegerLength[ n],3]]],k++];k]]; Array[snm,80] (* _Harvey P. Dale_, Aug 06 2024 *)
%Y A372262 Cf. A372056, A090584, A069568, A363922.
%K A372262 sign,base
%O A372262 1,13
%A A372262 _Toshitaka Suzuki_, Apr 24 2024