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A138154
Numbers n such that sigma(sigma(n))-phi(phi(n)) = 2n.
2
OFFSET
1,1
COMMENTS
No other term <10^6.
No other terms below 5*10^6 [From Jasper Mulder (jasper.mulder(AT)planet.nl), Jun 03 2010]
No other terms < 10^10. - Donovan Johnson, Feb 25 2013
a(4) > 1.5*10^12, if it exists. - Giovanni Resta, Nov 29 2019
EXAMPLE
3193 is in the sequence because sigma(sigma(3193))-phi(phi(3193)) = sigma(3328)-phi(3060) = 7154-768 = 6386 = 2*3193.
MAPLE
with(numtheory): a:=proc(n) if sigma(sigma(n))-phi(phi(n))=2*n then n else end if end proc: seq(a(n), n=1..1000000);
CROSSREFS
KEYWORD
more,nonn,bref
AUTHOR
Emeric Deutsch, Mar 03 2008
STATUS
approved