The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A253674 Indices of centered octagonal numbers (A016754) which are also centered triangular numbers (A005448). 3
 1, 10, 40, 931, 3871, 91180, 379270, 8934661, 37164541, 875505550, 3641745700, 85790609191, 356853914011, 8406604195120, 34968041827330, 823761420512521, 3426511245164281, 80720212606031890, 335763133984272160, 7909757073970612651, 32901360619213507351 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also positive integers y in the solutions to 3*x^2 - 8*y^2 - 3*x + 8*y = 0, the corresponding values of x being A253673. LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (1,98,-98,-1,1). FORMULA a(n) = a(n-1)+98*a(n-2)-98*a(n-3)-a(n-4)+a(n-5). G.f.: -x*(x^2-5*x+1)*(x^2+14*x+1) / ((x-1)*(x^2-10*x+1)*(x^2+10*x+1)). EXAMPLE 10 is in the sequence because the 10th centered octagonal number is 361, which is also the 16th centered triangular number. MATHEMATICA LinearRecurrence[{1, 98, -98, -1, 1}, {1, 10, 40, 931, 3871}, 30] (* Harvey P. Dale, Oct 01 2015 *) PROG (PARI) Vec(-x*(x^2-5*x+1)*(x^2+14*x+1)/((x-1)*(x^2-10*x+1)*(x^2+10*x+1)) + O(x^100)) CROSSREFS Cf. A005448, A016754, A253673, A253675. Sequence in context: A054885 A000449 A027274 * A016082 A003355 A247201 Adjacent sequences:  A253671 A253672 A253673 * A253675 A253676 A253677 KEYWORD nonn,easy AUTHOR Colin Barker, Jan 08 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.

Last modified November 26 09:38 EST 2020. Contains 338639 sequences. (Running on oeis4.)