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 A099080 #32 Feb 16 2025 08:32:54
%S A099080 2,3,66,102
%N A099080 Numbers k such that sigma(k).sigma(k-1) ... sigma(2).sigma(1) is prime (dot between numbers means concatenation).
%C A099080 Numbers of digits of primes corresponding to the four known terms of this sequence are respectively 2, 3, 133, and 232.
%C A099080 A naive heuristic suggests that this sequence is infinite but extremely sparse. - _Charles R Greathouse IV_, Nov 05 2013
%C A099080 There are no more terms below 10000. - _Charles R Greathouse IV_, Nov 09 2013
%C A099080 There are no more terms below 20000. - _Michael S. Branicky_, Nov 25 2024
%H A099080 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_008.htm">Puzzle 8. Primes by Listing</a>, The Prime Puzzles & Problems connection.
%H A099080 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IntegerSequencePrimes.html">Integer Sequence Primes</a>
%e A099080 3 is in the sequence because sigma(3).sigma(2).sigma(1) = 431 is prime.
%t A099080 Module[{nn=110,d},d=DivisorSigma[1,Range[nn]];Select[Range[nn], PrimeQ[ FromDigits[ Flatten[IntegerDigits/@Reverse[Take[d,#]]]]]&]] (* _Harvey P. Dale_, Jul 25 2016 *)
%o A099080 (PARI) s="1";for(n=2,1e3,s=Str(sigma(n),s);if(ispseudoprime(eval(s)), print1(n", "))) \\ _Charles R Greathouse IV_, Nov 05 2013
%Y A099080 Cf. A046035, A099077, A099078, A099079.
%K A099080 base,more,nonn
%O A099080 1,1
%A A099080 _Farideh Firoozbakht_, Oct 23 2004