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 A283561 #16 May 01 2025 01:36:21
%S A283561 1,2,5,550,832,10431
%N A283561 Numbers k such that the concatenation of the first k nonsquares gives a prime.
%C A283561 Indices n for which A283560(n) is prime.
%C A283561 A283560(1) = 2, A283560(2) = 23, A283560(5) = 23567, A283560(550) = 23567810...570571572573 is 1554-digits prime, A283560(832) = 23567810...858859860861 is 2400-digits prime.
%C A283561 Next term, if there is, will be more than 6100.
%C A283561 a(7) > 30000. - _Michael S. Branicky_, Apr 30 2025
%H A283561 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ConsecutiveNumberSequences.html">Consecutive Number Sequences</a>
%t A283561 cns[n_]:=FromDigits[Flatten[IntegerDigits[Table[k+Floor[1/2+Sqrt[k]],{k,1,n}]]]]
%t A283561 Select[Table[cns[n],{n,6100}],PrimeQ]
%o A283561 (PARI) is(n)=my(s=""); for(k=1,n, s=Str(s, (sqrtint(4*k)+1)\2 + k)); ispseudoprime(eval(s)) \\ _Charles R Greathouse IV_, Mar 10 2017
%Y A283561 Cf. A000037, A019518, A046035, A283560, A283802.
%K A283561 nonn,base,more
%O A283561 1,2
%A A283561 _XU Pingya_, Mar 10 2017
%E A283561 a(6) from _Michael S. Branicky_, Apr 28 2025