A260144 - OEIS

%I #19 Jul 14 2021 09:18:14

%S 1568,14049280,140492800,368089904,506300928,1404928000

%N Numbers k such that there exist two numbers a and b where k = a.b = sigma(a)*phi(b) ("." means concatenation).

%C The sequence is infinite because, for m>0, 14049280*10^m = sigma(1404) * phi(9280*10^m).

%C a(7) > 10^10.

%e 1568 is in the sequence because 1568 = sigma(156) * phi(8).

%Y Cf. A000010, A000203, A159000, A253824.

%K nonn,base,more

%O 1,1

%A _Paolo P. Lava_ and _Giovanni Resta_, Jul 17 2015