The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A136410 Numbers n having a proper divisor d > 2 such that d-1 divides n-1. 1
 9, 15, 16, 21, 25, 27, 28, 33, 36, 39, 40, 45, 49, 51, 52, 57, 63, 64, 65, 66, 69, 75, 76, 81, 85, 87, 88, 91, 93, 96, 99, 100, 105, 111, 112, 117, 120, 121, 123, 124, 125, 126, 129, 133, 135, 136, 141, 144, 145, 147, 148 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS There is a triangular array of n dots, having at least three rows, having row sizes 1, 1+2x, 1+4x, 1+6x, ... iff n is in this sequence (where x equals all the natural numbers). - Peter Woodward, Apr 24 2015 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE E.g. consider n=91: we can take d=7, 7 divides 91 and 6 divides 90, so 91 is in the sequence. MAPLE N:= 1000: # to get all terms <= N {seq(seq(d+k*d*(d-1), k=1..floor((N-d)/d/(d-1))), d=3..floor(sqrt(N)))}; # if using Maple 11 or earlier, uncomment the next line # sort(convert(%, list));  # Robert Israel, Apr 24 2015 MATHEMATICA fQ[n_] := Block[{d = Select[ Take[ Divisors@ n, {2, -2}], # > 2 &]}, Union[IntegerQ /@ ((n - 1)/(d - 1))][[ -1]]]; Select[ Range@ 175, !PrimeQ@ # && fQ@ # &] (* Robert G. Wilson v, May 04 2008 *) CROSSREFS Sequence in context: A058957 A257409 A105882 * A324879 A066942 A257048 Adjacent sequences:  A136407 A136408 A136409 * A136411 A136412 A136413 KEYWORD nonn AUTHOR J. Perry (johnandruth(AT)jrperry.orangehome.co.uk), Apr 13 2008 EXTENSIONS Definition, terms and offset corrected by M. F. Hasler, May 01 2008 Edited by N. J. A. Sloane, May 10 2008 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.

Last modified January 28 13:19 EST 2020. Contains 331321 sequences. (Running on oeis4.)