login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A326181 Numbers n for which sigma(sigma(n)) = 3*sigma(n). 5
54, 56, 87, 95, 276, 308, 429, 446, 455, 501, 581, 611, 158928, 194928, 195072, 199950, 226352, 234608, 236432, 248325, 255678, 263504, 266192, 273050, 275415, 304575, 336903, 341162, 353675, 366575, 369425, 369843, 380463, 386313, 389463, 406565, 411725, 415925, 422303, 447587, 468743, 497333, 500993, 511829, 515267, 519557, 519677 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Any odd perfect numbers must occur in this sequence, as such numbers must be in the intersection of A000396 and A326051, that is, satisfy both sigma(n) = 2n and sigma(2n) = 6n = 3*2n, thus in combination they must satisfy sigma(sigma(n)) = 3*sigma(n). Note that odd perfect numbers should occur also in A019283.

If, as conjectured, A005820 has 6 terms, then this sequence is finite and has 756 terms. - Giovanni Resta, Jun 17 2019

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..756 (first 169 terms from Antti Karttunen)

Index entries for sequences where any odd perfect numbers must occur

PROG

(PARI) isA326181(n) = { my(s=sigma(n)); (sigma(s)==3*s); };

CROSSREFS

Cf. A000203, A000396, A005820, A019283, A051027, A087943, A272027 (3*sigma(n)), A326051.

Subsequence of A066961.

Sequence in context: A116386 A107936 A252722 * A300447 A344809 A344810

Adjacent sequences:  A326178 A326179 A326180 * A326182 A326183 A326184

KEYWORD

nonn

AUTHOR

Antti Karttunen, Jun 16 2019

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 January 28 15:20 EST 2022. Contains 350657 sequences. (Running on oeis4.)