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!)
A129297 Nonnegative integers m such that m^2-1 has no divisors d with 1<d<m-1. 6
0, 1, 2, 3, 4, 6, 12, 18, 30, 42, 60, 72, 102, 108, 138, 150, 180, 192, 198, 228, 240, 270, 282, 312, 348, 420, 432, 462, 522, 570, 600, 618, 642, 660, 810, 822, 828, 858, 882, 1020, 1032, 1050, 1062, 1092, 1152, 1230, 1278, 1290, 1302, 1320, 1428, 1452, 1482, 1488 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Since m^2-1 = (m+1)(m-1), this sequence is just 0,1,2,3, and the average of twin prime pairs A014574.
LINKS
FORMULA
A129296(a(n)) = #{1, a(n)-1} = 2;
a(n) = A014574(n-4) for n>4.
EXAMPLE
{1,41,43,1763} is the set of divisors of 42^2-1, therefore 42 is a term, A129296(42) = #{1,41} = 2.
MATHEMATICA
nniQ[n_]:=Count[Rest[Divisors[n^2-1]], _?(#<n-1&)]==0; Join[{0, 1}, Select[ Range[2, 1500], nniQ]] (* Harvey P. Dale, Aug 09 2015 *)
Select[Range[0, 1500], #<4 || And @@ PrimeQ[# + {-1, 1}] &] (* Amiram Eldar, Jun 18 2022 *)
PROG
(PARI) isA129297(n) = (n <= 3) || divisors(n^2-1)[2] >= n-1
CROSSREFS
Sequence in context: A048330 A118651 A206292 * A337449 A221846 A018343
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, Apr 09 2007
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 March 29 11:45 EDT 2024. Contains 371278 sequences. (Running on oeis4.)