login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A176774 Smallest polygonality of n = smallest integer m>=3 such that n is m-gonal number. 18
3, 4, 5, 3, 7, 8, 4, 3, 11, 5, 13, 14, 3, 4, 17, 7, 19, 20, 3, 5, 23, 9, 4, 26, 10, 3, 29, 11, 31, 32, 12, 7, 5, 3, 37, 38, 14, 8, 41, 15, 43, 44, 3, 9, 47, 17, 4, 50, 5, 10, 53, 19, 3, 56, 20, 11, 59, 21, 61, 62, 22, 4, 8, 3, 67, 68, 24, 5, 71, 25, 73, 74, 9, 14, 77, 3, 79, 80, 4, 15, 83 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,1

COMMENTS

A176775(n) gives the index of n as a(n)-gonal number.

Since n is the second n-gonal number, a(n) <= n.

Furthermore, a(n)=n iff A176775(n)=2.

LINKS

Michel Marcus, Table of n, a(n) for n = 3..10000

Eric W. Weisstein, Polygonal Number. MathWorld.

EXAMPLE

a(12) = 5 since 12 is a pentagonal number, but not a square or triangular number. - Michael B. Porter, Jul 16 2016

MATHEMATICA

a[n_] := (m = 3; While[Reduce[k >= 1 && n == k (k (m - 2) - m + 4)/2, k, Integers] == False, m++]; m); Table[a[n], {n, 3, 100}] (* Jean-François Alcover, Sep 04 2016 *)

PROG

(PARI) a(n) = {k=3; while (! ispolygonal(n, k), k++); k; } \\ Michel Marcus, Mar 25 2015

(Python)

from __future__ import division

from gmpy2 import isqrt

def A176774(n):

k = (isqrt(8*n+1)-1)//2

while k >= 2:

a, b = divmod(2*(k*(k-2)+n), k*(k-1))

if not b:

return a

k -= 1 # Chai Wah Wu, Jul 28 2016

CROSSREFS

Cf. A090466, A090467.

Sequence in context: A349164 A214682 A093395 * A126352 A354998 A094758

Adjacent sequences: A176771 A176772 A176773 * A176775 A176776 A176777

KEYWORD

nonn

AUTHOR

Max Alekseyev, Apr 25 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 5 17:04 EST 2022. Contains 358588 sequences. (Running on oeis4.)