A260144 Numbers k such that there exist two numbers a and b where k = a.b = sigma(a)*phi(b) ("." means concatenation).

1568, 14049280, 140492800, 368089904, 506300928, 1404928000, 14049280000, 124856008704, 140492800000, 1404928000000

Offset: 1

Author

Paolo P. Lava and Giovanni Resta, Jul 17 2015

Keywords

• nonn
• base
• more

Comments

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

Examples

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

Crossrefs

Cf. A000010, A000203, A159000, A253824.

Extensions

a(7)-a(10) from Max Alekseyev, May 26 2025