login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A180248 Odd composite squarefree numbers n such that r = 2*(d-2+n/d)/(d-1) is an integer for each prime divisor d of n. 1
15, 91, 435, 561, 703, 1105, 1729, 1891, 2465, 2701, 2821, 3367, 5551, 6601, 8695, 8911, 10585, 11305, 12403, 13981, 15051, 15841, 16471, 18721, 23001, 26335, 29341, 30889, 38503, 39865, 41041, 46657, 49141, 52633, 53131, 62745 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: n is a Carmichael number (A002997) if and only if n is a term of A180248 and all r-values of n are even.

LINKS

K. Brockhaus, Table of n, a(n) for n = 1..653 (terms < 10^8)

PROG

(PARI) isok(n) = {if (((n % 2)==0) || isprime(n) || !issquarefree(n), return (0)); f = factor(n); for (i=1, #f~, d = f[i, 1]; if (type(2*(d-2+n/d)/(d-1)) != "t_INT", return(0)); ); return (1); } \\ Michel Marcus, Jul 12 2013

CROSSREFS

Sequence in context: A237516 A020242 A020255 * A041428 A052226 A108684

Adjacent sequences:  A180245 A180246 A180247 * A180249 A180250 A180251

KEYWORD

nonn

AUTHOR

William F. Sindelar (w_sindelar(AT)juno.com), Aug 19 2010

EXTENSIONS

Edited by the Associate Editors of the OEIS, Sep 04 2010

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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified June 24 07:19 EDT 2017. Contains 288697 sequences.