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A062516 Numbers n such that tau(n)*2 = phi(n). 12
5, 9, 15, 28, 40, 72, 84, 90, 120 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Sequence is finite, since for large n and suitable constants and epsilon: phi(n)-2*Tau[n]>c1*n^(2/3)-4c2*n^(1/2)>0 if n>c3, so phi(n)-2*Tau[n]>0, QED. Moreover, phi(n)=k*Tau[n] has at most finitely many solutions for any constant k or even for slowly increasing functions like k(n)=n^(epsilon). - Labos Elemer, Jul 20 2001

LINKS

Table of n, a(n) for n=1..9.

PROG

(PARI) for(n=1, 1000000, if(numdiv(n)*2==eulerphi(n), print(n), ))

CROSSREFS

Cf. A112954, A020488, A063469, A063470.

Sequence in context: A023498 A256343 A210289 * A249331 A075133 A066081

Adjacent sequences:  A062513 A062514 A062515 * A062517 A062518 A062519

KEYWORD

nonn,fini,full

AUTHOR

Jason Earls, Jul 13 2001

EXTENSIONS

"full" keyword from Max Alekseyev, Mar 01 2010

STATUS

approved

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Last modified April 9 12:17 EDT 2020. Contains 333348 sequences. (Running on oeis4.)