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!)
A073586 Composite numbers k such that (k+1)*sigma(k) is a perfect square. 1
188, 1155, 2540, 5831, 6560, 8449, 9074, 11235, 11448, 25885, 32318, 54144, 56168, 61016, 63535, 76879, 91874, 99980, 118335, 119144, 123100, 127007, 146015, 153759, 219699, 227528, 234255, 243048, 280370, 404319, 405768, 408968, 431648, 463346, 504007 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Trivially if p is prime then (p+1)*sigma(p) = (p+1)^2.

LINKS

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

EXAMPLE

1156*sigma(1155) = 1632^2, hence 1155 is in the sequence.

MATHEMATICA

Select[Range[505000], ! PrimeQ[#] && IntegerQ[Sqrt[(# + 1)*DivisorSigma[1, #]]] &] (* Jayanta Basu, Jul 02 2013 *)

PROG

(PARI) isA073586(n)=(!isprime(n))&&issquare((n+1)*sigma(n)) \\ Michael B. Porter, Apr 05 2010

CROSSREFS

Cf. A069953.

Sequence in context: A335592 A233786 A233961 * A064115 A185241 A211818

Adjacent sequences:  A073583 A073584 A073585 * A073587 A073588 A073589

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Aug 28 2002

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 July 30 10:27 EDT 2021. Contains 346359 sequences. (Running on oeis4.)