A251362 - OEIS

%I #22 Feb 11 2025 23:33:56

%S 25,235741,23517131,274873357929,2357131984859

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

%C Numbers n such that n = A084317(A000010(n)). - _Michel Marcus_, Dec 06 2014

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

%e 235741 is in the sequence since phi(235741)=2^4*3^2*5*7*41,

%e 23517131 is in the sequence since phi(23517131)=2^7*3*5^2*17*131.

%t a251362[n_Integer] := Rest@ Select[Range[n], # ==

%t FromDigits[Flatten@IntegerDigits[First@Transpose@FactorInteger[EulerPhi[#]]]] &]; a251362[10^6] (* _Michael De Vlieger_, Dec 03 2014 *)

%Y Cf. A000010, A000040, A084317, A251360, A251361, A251363, A329026.

%K nonn,base,more

%O 1,1

%A _Jahangeer Kholdi_, Dec 03 2014

%E a(4)-a(5) from _Max Alekseyev_, Feb 11 2025