login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A015713 phi(n) * sigma(n) + k^2 is not a square for any k. 1
4, 9, 18, 49, 81, 98, 121, 162, 242, 361, 529, 722, 729, 961, 1058, 1458, 1849, 1922, 2209, 2401, 3481, 3698, 4418, 4489, 4802, 5041, 6241, 6561, 6889, 6962, 8978, 10082, 10609, 11449, 12482, 13122, 13778, 14641, 16129, 17161 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers n such that A062354(n) is in A016825. - Michel Marcus, Dec 07 2018

LINKS

Table of n, a(n) for n=1..40.

Richard K. Guy, Divisors and desires, Amer. Math. Monthly, 104 (1997), 359-360.

FORMULA

Conjecture: {4, p^(2*m), 2*p^(2*m), p = 4*k+3 is prime}. - Sean A. Irvine, Dec 06 2018

MATHEMATICA

nonSqDiffQ[n_] := Mod[n, 4] == 2; aQ[n_] := nonSqDiffQ[ EulerPhi[n] * DivisorSigma[ 1, n]]; Select[Range[20000], aQ] (* Amiram Eldar, Dec 07 2018 *)

PROG

(PARI) isok(n) = (sigma(n)*eulerphi(n) % 4) == 2; \\ Michel Marcus, Dec 07 2018

CROSSREFS

Cf. A015710, A062354 (phi(n)*sigma(n)), A016825.

Sequence in context: A229072 A261983 A074896 * A049198 A146303 A203205

Adjacent sequences:  A015710 A015711 A015712 * A015714 A015715 A015716

KEYWORD

nonn

AUTHOR

Robert G. Wilson v

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 20 04:04 EDT 2020. Contains 337264 sequences. (Running on oeis4.)