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

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A256150 Oblong numbers n such that sigma(n) is a triangular number. 3
2, 12, 56, 342, 992, 16256, 17822, 169332, 628056, 1189190, 2720850, 11085570, 35599122, 67100672, 1147210770, 1317435912, 1707135806, 7800334080, 11208986256, 13366943840, 17109032402, 17179738112, 46343540900, 58413331032, 83717924940, 204574837700, 274877382656, 445968192672, 589130699852 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The numbers 12, 56, 992, 16256, 67100672,…( A139256(n)), twice even perfect numbers, are in the sequence because are oblong (A139256(n)=2^k*(2^k-1) with 2^k-1 Mersenne prime) and sigma(A139256(n))=sigma(2^k*(2^k-1))=sigma(2^k )*sigma( (2^k-1)=(2^(k+1)-1)*2^(k+1)/2, triangular number.

This sequence is the intersection of A002378 and A045746.

LINKS

Table of n, a(n) for n=1..29.

EXAMPLE

2 is in the sequence because 2=1*2 is oblong, and sigma(2)= 1+2=3=2*3/2 is triangular.

MATHEMATICA

Select[2 Accumulate[Range@10000], MemberQ[Accumulate[Range@10000], DivisorSigma[1, #]] &] (* Michael De Vlieger, Mar 17 2015 *)

PROG

(PARI) {for (i=1, i=10^6, n=i*(i+1); if(ispolygonal(sigma(n), 3), print(n)))}

CROSSREFS

Cf. A000217, A002378, A139256, A045746, A256149, A256151, A256152.

Sequence in context: A296944 A105487 A098453 * A180073 A067125 A177782

Adjacent sequences:  A256147 A256148 A256149 * A256151 A256152 A256153

KEYWORD

nonn

AUTHOR

Antonio Roldán, Mar 16 2015

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 December 13 09:58 EST 2019. Contains 329968 sequences. (Running on oeis4.)