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 A323532 #31 Sep 26 2019 11:07:51
%S A323532 10,586,2219
%N A323532 Numbers k such that the decimal concatenation of the numbers from 1 up to k followed by digit reversals of the numbers from (k-1) down to 1 is a prime.
%C A323532 The definition is related to the sequence discussed by N. J. A. Sloane (in Notices of the AMS (2018), Vol. 65, No. 9, pp. 1070-71) for which only a(1)-a(2) are known.
%C A323532 a(1) corresponds to a memorable prime (12345678910987654321); a(4) > 10000 (if it exists).
%H A323532 N. J. A. Sloane, <a href="https://www.ams.org/journals/notices/201809/rnoti-p1062.pdf">The On-Line Encyclopedia of Integer Sequences</a>, Notices, Amer. Math. Soc., 65 (No. 9, Oct. 2018), 1062-1074.
%e A323532 10 is a term because 12345678910987654321 is a prime.
%e A323532 2219 is a term because 1...22172218221981227122...1 is a 15534-digit probable prime (where 8122 following 2219 corresponds to the digit reversal of 2218, 7122 to that of 2217, etc. down to 1).
%t A323532 a[n_]:=Block[{cn=Drop[FoldList[Append, {}, ToString/@Range@n], 2]}, ParallelMap[If[PrimeQ[FromDigits@@{#<>Reverse@StringReverse@Most@#}], Length@#, Nothing]&, cn]]; a[2300]
%o A323532 (PARI) f(n) = eval(concat(vector(2*n-1, k, if(k<=n, Str(k), concat(apply(x->Str(x), Vecrev(digits(2*n-k))))))));
%o A323532 isok(n) = ispseudoprime(f(n)); \\ _Michel Marcus_, Jan 20 2019
%Y A323532 Cf. A173426 (similar but different concatenation scheme).
%K A323532 nonn,base,more
%O A323532 1,1
%A A323532 _Mikk Heidemaa_, Jan 17 2019