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A230032 Numbers n such that phi(sigma(n)) + sigma(phi(n)) < n. 2
138594, 249474, 277194, 471234, 554394, 665274, 900870, 1015554, 1081074, 1191954, 1244874, 1358274, 1385994, 1607754, 1801794, 1857234, 2189874, 2356170, 2356194, 2411634, 2439354, 2489754, 2522514, 2550234, 2633394, 2688834, 2702670, 2716554 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Let ps(n) be number of terms of the sequence up to n, it seems that ps(n) ~ n/100000. Is it true that 6 divides each term of the sequence?

I guess that there is no number n such that phi(sigma(n)) + sigma(phi(n)) = n.

From M. F. Hasler, Oct 31 2013: (Start)

Most terms of the sequence are of the form given in the following

Theorem: If p is a safe prime (A005385), then n = 6p is a term of this sequence if and only if (1-1/q1)*...*(1-1/qr) + 7/12 < p/(p+1), where q1,...,qr are the distinct odd prime factors of p+1.

Proof: Write p+1 = 2^a 3^b Q with gcd(Q,6)=1 and assume (p-1)/2 is prime. For n = 6p, an easy calculation yields phi(sigma(n)) + sigma(phi(n)) =  n*(1+1/p)*(2/3*(1-1/q2)*...*(1-1/qr)+7/12), where q2,...,qr are the prime factors of Q. #

Corollary: n=6p is in the sequence when p is a safe prime and p+1 is a multiple of 2*3*5*7*11 or of 2*3*5*7*13*q with some prime q>13, q<80. (End)

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..1000

MATHEMATICA

Do[If[EulerPhi[DivisorSigma[1, n]] + DivisorSigma[1, EulerPhi[n]] < n, Print[n]], {n, 3300000}]

PROG

(PARI) is_A230032(n)={eulerphi(sigma(n))+sigma(eulerphi(n))<n} \\ M. F. Hasler, Oct 31 2013

CROSSREFS

Cf. A000010, A000203, A062401, A062402, A066950.

Sequence in context: A236662 A251979 A224586 * A190386 A236276 A250713

Adjacent sequences:  A230029 A230030 A230031 * A230033 A230034 A230035

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Oct 28 2013

STATUS

approved

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Last modified February 19 21:59 EST 2019. Contains 320328 sequences. (Running on oeis4.)