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 A251361 #20 May 17 2025 08:16:01 %S A251361 4,100,31509,7560625 %N A251361 Numbers k such that pi(k) is the concatenation of distinct prime factors of k, in increasing order. %C A251361 Next term must be greater than 4*10^8. %C A251361 Numbers k such that A000720(k) = A084317(k). - _Michel Marcus_, Dec 06 2014 %e A251361 4 is in the sequence since 4=2^2 and pi(4)=2, %e A251361 100 is in the sequence since 100=2^2*5^2 and pi(100)=25, %e A251361 31509 is in the sequence since 31509=3^4*389 and pi(31509)=3389, and %e A251361 7560625 is in the sequence since 7560625=5^4*12097 and pi(7560625)=512097. %t A251361 a251361[n_Integer] := Select[Range[n], PrimePi[#] == FromDigits[ %t A251361 Flatten@ IntegerDigits[First@ Transpose@ FactorInteger[#]]] &]; a251361[10^6] (* _Michael De Vlieger_, Dec 03 2014 *) %o A251361 (PARI) is(n)=eval(fold((x,y)->Str(x,y),factor(n)[,1]))==primepi(n) \\ _Charles R Greathouse IV_, Dec 06 2014 %Y A251361 Cf. A000040, A000720, A084317, A251360. %K A251361 nonn,base,more %O A251361 1,1 %A A251361 _Jahangeer Kholdi_, Dec 02 2014 %E A251361 Definition corrected by _Max Alekseyev_, Feb 12 2025