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.
5 is in the sequence because pi(1).pi(2).pi(3).pi(4).pi(5)=1223 is prime.
5 is in the sequence because pi(5).pi(4).pi(3).pi(2) = 3221 is prime.
r:= 1: v:= 1: Res:= NULL: for k from 3 to 6000 do if isprime(k) then r:= r+1 fi; v:= v + r*10^(1+ilog10(v)); if isprime(v) then Res:= Res, k fi od: Res; # Robert Israel, Nov 20 2018
s = ""; Do[s = ToString[PrimePi[n]] <> s; k = ToExpression[s]; If[PrimeQ[k], Print[n]], {n, 2, 5235}] (* Ryan Propper, Aug 30 2005 *)
9 is in the sequence because phi(9).phi(8).phi(7).phi(6).phi(5).phi(4).phi(3).phi(2).phi(1) = 646242211 is prime.
Module[{nn=210,eph},eph=EulerPhi[Range[nn]];Position[Table[FromDigits[ Flatten[ IntegerDigits[Reverse[Take[eph,n]]]]],{n,nn}],?PrimeQ]]// Flatten (* _Harvey P. Dale, Apr 21 2020 *)
ParallelTable[If[PrimeQ[ToExpression[StringJoin[ToString[#]&/@Reverse[Table[EulerPhi[k],{k,1,n}]]]]],n,Nothing],{n,1,10^4}]//.{}->Nothing (* J.W.L. (Jan) Eerland, Aug 15 2022 *)
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