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A138154 Numbers n such that sigma(sigma(n))-phi(phi(n)) = 2n. 2
3, 5, 3193 (list; graph; refs; listen; history; text; internal format)
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

LINKS

Table of n, a(n) for n=1..3.

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

Cf. A136131, A136132, A137600.

Sequence in context: A012783 A062655 A090807 * A333269 A319598 A126334

Adjacent sequences:  A138151 A138152 A138153 * A138155 A138156 A138157

KEYWORD

more,nonn,bref

AUTHOR

Emeric Deutsch, Mar 03 2008

STATUS

approved

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Last modified October 17 14:54 EDT 2021. Contains 348049 sequences. (Running on oeis4.)