A251361 Numbers k such that pi(k) is the concatenation of distinct prime factors of k, in increasing order.

4, 100, 31509, 7560625

Offset: 1

Author

Jahangeer Kholdi, Dec 02 2014

Keywords

• nonn
• base
• more

Comments

Next term must be greater than 4*10^8.

Numbers k such that A000720(k) = A084317(k). - Michel Marcus, Dec 06 2014

Examples

			4 is in the sequence since 4=2^2 and pi(4)=2,
100 is in the sequence since 100=2^2*5^2 and pi(100)=25,
31509 is in the sequence since 31509=3^4*389 and pi(31509)=3389, and
7560625 is in the sequence since 7560625=5^4*12097 and pi(7560625)=512097.
		

Crossrefs

Cf. A000040, A000720, A084317, A251360.

Programs

Mathematica

a251361[n_Integer] := Select[Range[n], PrimePi[#] == FromDigits[
Flatten@ IntegerDigits[First@ Transpose@ FactorInteger[#]]] &]; a251361[10^6] (* Michael De Vlieger, Dec 03 2014 *)

PARI

is(n)=eval(fold((x,y)->Str(x,y),factor(n)[,1]))==primepi(n) \\ Charles R Greathouse IV, Dec 06 2014

Extensions

Definition corrected by Max Alekseyev, Feb 12 2025