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 A283801 #16 Feb 16 2025 08:33:43
%S A283801 9,915,91521,9152125,915212527,91521252733,9152125273335,
%T A283801 915212527333539,91521252733353945,9152125273335394549,
%U A283801 915212527333539454951,91521252733353945495155,9152125273335394549515557,915212527333539454951555763,91521252733353945495155576365
%N A283801 Concatenation of the first n odd composite numbers (A071904).
%C A283801 There are 3 primes in the first 5028 terms of this sequence, see A283802.
%H A283801 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ConsecutiveNumberSequences.html">Consecutive Number Sequences</a>
%t A283801 bb[1]=9;bb[n_]:=bb[n]=Which[PrimeQ[bb[n-1]+2]==False,bb[n-1]+2,PrimeQ[bb[n-1]+4]==False,bb[n-1]+4,True,bb[n-1]+6];coc[n_]:=FromDigits[Flatten[IntegerDigits[Table[bb[k],{k,1,n}]]]];Table[coc[n],14]
%t A283801 f[n_] := Block[{oc = cc = 0, k = 2}, While[oc <= n, If[ OddQ@ k && !PrimeQ@ k, cc = cc*10^IntegerLength[k] +k; oc++]; k++]; cc]; Array[f, 14] (* _Robert G. Wilson v_, Mar 17 2017 *)
%Y A283801 Cf. A071904, A089933, A132932.
%K A283801 base,nonn,easy
%O A283801 1,1
%A A283801 _XU Pingya_, Mar 17 2017