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A260144
Numbers k such that there exist two numbers a and b where k = a.b = sigma(a)*phi(b) ("." means concatenation).
2
1568, 14049280, 140492800, 368089904, 506300928, 1404928000
OFFSET
1,1
COMMENTS
The sequence is infinite because, for m>0, 14049280*10^m = sigma(1404) * phi(9280*10^m).
a(7) > 10^10.
EXAMPLE
1568 is in the sequence because 1568 = sigma(156) * phi(8).
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Paolo P. Lava and Giovanni Resta, Jul 17 2015
STATUS
approved