The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
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!)
A081380 Numbers n such that the sets of prime factors (ignoring multiplicity) of A000203(n) = sigma(n) and of A001157(n) = sigma_2(n) are identical. 2
1, 180, 1444, 12996, 23805, 36100, 52020, 60228, 64980, 68832, 95220, 301140, 324900, 344160, 481824, 1505700, 1718721, 1720800, 2275758, 2409120, 3755844, 6874884, 6879645, 7965153, 8593605, 11378790, 12045600, 15930306, 17405892 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
REFERENCES
J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 180, p. 56, Ellipses, Paris 2008.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..67 (terms < 2^31)
EXAMPLE
n = 1444 = 2^2*19^2, sigma(1444) = 2667 = 3*7*127, sigma_2(1444) = 2744343 = 3^2*7^4*127, common factor set = {3,7,127}.
MATHEMATICA
ffi[x_] := Flatten[FactorInteger[x]] lf[x_] := Length[FactorInteger[x]] ba[x_] := Table[Part[ffi[x], 2*w-1], {w, 1, lf[x]}] Do[s=ba[DivisorSigma[1, n]]; s5=ba[DivisorSigma[2, n]]; If[Equal[s, s5], Print[n]], {n, 1, 1000000}]
PROG
(PARI) is(n)=factor(sigma(n))[, 1]==factor(sigma(n, 2))[, 1] \\ Charles R Greathouse IV, Feb 19 2013
CROSSREFS
Sequence in context: A251204 A225933 A225932 * A115184 A154672 A211556
KEYWORD
nonn
AUTHOR
Labos Elemer, Mar 26 2003
EXTENSIONS
More terms from Lekraj Beedassy, Jul 18 2008
a(16)-a(29) from Donovan Johnson, May 24 2009
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 16 19:52 EDT 2024. Contains 373432 sequences. (Running on oeis4.)