A260021 - OEIS

A260021

Solutions to phi(n) = phi(sigma(n)) that are not given by Theorem 3 of Golomb's manuscript.

1, 3, 15, 45, 175, 357, 585, 608, 646, 962, 1071, 1292, 1443, 1508, 1586, 1664, 1665, 1898, 2275, 2295, 2379, 2745, 2847, 3285, 3848, 4082, 4329, 4514, 4641, 4736, 4845, 5018, 5402, 6123, 6232, 6344, 6475, 6771, 7052, 7065, 7137, 7202, 7215, 7527, 7592, 7803, 7808, 8103, 8138, 8398, 8541, 8685, 8906, 9344, 9526, 10322

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

REFERENCES

S. W. Golomb, Equality among number-theoretic functions, Manuscript, no date; Second update, Dec 29, 1992.

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..20000

S. W. Golomb, Equality among number-theoretic functions, Unpublished manuscript. (Annotated scanned copy)

Antti Karttunen, Data supplement: n, a(n) computed for n = 1..103800

Index entries for sequences related to sigma(n)

FORMULA

{k | 1==A353637(k) and 0==A354344(k)}. - Antti Karttunen, May 25 2022

PROG

(PARI)

A354344(n) = { if(!(n%15), n/=15, if(!(n%9), n/=9, if(!(n%8), n/=8, if(!(n%3), n/=3, if(!(n%2), n/=2, return(0)))))); ((n>5) && isprime(n) && isprime((1+n)/2)); };

A353637(n) = (eulerphi(sigma(n))==eulerphi(n));

isA260021(n) = (A353637(n) && !A354344(n)); \\ Antti Karttunen, May 24 2022

CROSSREFS

Setwise difference A006872 \ A354345. Subset of positions of zeros in A353636.

Cf. A354362 (subsequence).

Cf. also A005383, A353637, A354344.

Sequence in context: A301632 A074355 A201868 * A005560 A100747 A100737

Adjacent sequences: A260018 A260019 A260020 * A260022 A260023 A260024

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Jul 14 2015

EXTENSIONS

Term a(1) = 1 prepended and terms a(14) .. a(56) added by Antti Karttunen, May 24 2022

STATUS

approved