A251361 - OEIS

A251361

Numbers n such that pi(n) is the concatenation of distinct prime factors of n, in decreasing order.

4, 100, 31509, 7560625

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OFFSET

1,1

COMMENTS

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

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

LINKS

EXAMPLE

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.

MATHEMATICA

a251361[n_Integer] := Select[Range[n], PrimePi[#] == FromDigits[

Flatten@ IntegerDigits[First@ Transpose@ FactorInteger[#]]] &]; a251361[10^6] (* Michael De Vlieger, Dec 03 2014 *)

PROG

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

CROSSREFS

KEYWORD

nonn,base,more

AUTHOR

STATUS

approved