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

 

Logo

Thanks to everyone who made a donation during our annual appeal!
To see the list of donors, or make a donation, see the OEIS Foundation home page.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A195527 Integers n that are k-gonal for precisely 3 distinct values of k, where k >= 3. 7
15, 21, 28, 51, 55, 64, 70, 75, 78, 91, 96, 100, 111, 112, 117, 126, 135, 136, 141, 144, 145, 148, 154, 156, 165, 175, 176, 186, 189, 195, 201, 204, 216, 232, 235, 238, 246, 255, 256, 285, 286, 288, 291, 297, 300, 306, 315, 316, 321, 322, 324, 330, 333, 336 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

See A177025 for number of ways a number can be represented as a polygonal number.

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..10000

EXAMPLE

21 is in the sequence because it is a triangular number (A000217), an octagonal number (A000567) and an icosihenagonal number (A051873).

MATHEMATICA

data1=Reduce[1/2 n (n(k-2)+4-k)== # && k>=3 && n>0, {k, n}, Integers]&/@Range[336]; data2=If[Head[#]===And, 1, Length[#]] &/@data1; data3=DeleteCases[Table[If[data2[[k]]==3, k], {k, 1, Length[data2]}], Null]

PROG

(Python)

A195527_list = []

for m in range(1, 10**4):

    n, c = 3, 0

    while n*(n+1) <= 2*m:

        if not 2*(n*(n-2) + m) % (n*(n - 1)):

            c += 1

            if c > 2:

                break

        n += 1

    if c == 2:

        A195527_list.append(m) # Chai Wah Wu, Jul 28 2016

CROSSREFS

Cf. A000217, A000567, A051873, A177025, A177029.

Sequence in context: A102030 A168104 A026048 * A047200 A070537 A324110

Adjacent sequences:  A195524 A195525 A195526 * A195528 A195529 A195530

KEYWORD

nonn

AUTHOR

Ant King, Sep 21 2011

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 22 22:16 EST 2020. Contains 331166 sequences. (Running on oeis4.)