A251361 - OEIS

A251361

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

%I #15 Dec 06 2014 23:53:05

%S 4,100,31509,7560625

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

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

%C Numbers n such that A000720(n) = A084317(n). - _Michel Marcus_, Dec 06 2014.

%e 4 is in the sequence since 4=2^2 and pi(4)=2,

%e 100 is in the sequence since 100=2^2*5^2 and pi(100)=25,

%e 31509 is in the sequence since 31509=3^4*389 and pi(31509)=3389, and

%e 7560625 is in the sequence since 7560625=5^4*12097 and pi(7560625)=512097.

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

%t Flatten@ IntegerDigits[First@ Transpose@ FactorInteger[#]]] &]; a251361[10^6] (* _Michael De Vlieger_, Dec 03 2014 *)

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

%Y Cf. A000040, A000720, A084317, A251360.

%K nonn,base,more

%O 1,1

%A _Jahangeer Kholdi_, Dec 02 2014