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%I #21 Apr 30 2023 11:19:47
%S 5,22,48,317,734,5235,12377
%N Numbers k such that pi(k).pi(k-1) ... pi(3).pi(2) is prime (dot between numbers means concatenation).
%C Number of digits of primes corresponding to the five known terms of this sequence are respectively 4, 21, 67, 605, 1633.
%H Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_008.htm">Puzzle 8. Primes by Listing</a>, The Prime Puzzles & Problems connection.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/IntegerSequencePrimes.html">Integer Sequence Primes</a>
%e 5 is in the sequence because pi(5).pi(4).pi(3).pi(2) = 3221 is prime.
%p r:= 1: v:= 1: Res:= NULL:
%p for k from 3 to 6000 do
%p if isprime(k) then r:= r+1 fi;
%p v:= v + r*10^(1+ilog10(v));
%p if isprime(v) then Res:= Res, k fi
%p od:
%p Res; # _Robert Israel_, Nov 20 2018
%t s = ""; Do[s = ToString[PrimePi[n]] <> s; k = ToExpression[s]; If[PrimeQ[k], Print[n]], {n, 2, 5235}] (* _Ryan Propper_, Aug 30 2005 *)
%Y Cf. A046035, A099077, A099079, A099080.
%K base,more,nonn
%O 1,1
%A _Farideh Firoozbakht_, Oct 23 2004
%E a(6) from _Ryan Propper_, Aug 30 2005
%E a(7) from _Michael S. Branicky_, Apr 29 2023
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