The OEIS is supported by the many generous donors to the OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A005891 Centered pentagonal numbers: (5n^2+5n+2)/2; crystal ball sequence for 3.3.3.4.4. planar net. (Formerly M4112) 82
 1, 6, 16, 31, 51, 76, 106, 141, 181, 226, 276, 331, 391, 456, 526, 601, 681, 766, 856, 951, 1051, 1156, 1266, 1381, 1501, 1626, 1756, 1891, 2031, 2176, 2326, 2481, 2641, 2806, 2976, 3151, 3331, 3516, 3706, 3901, 4101, 4306, 4516, 4731, 4951, 5176, 5406 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Equals the triangular numbers convolved with [1, 3, 1, 0, 0, 0, ...]. - Gary W. Adamson and Alexander R. Povolotsky, May 29 2009 From Ant King, Jun 15 2012: (Start) a(n) == 1 (mod 5) for all n. The digital roots of the a(n) form a purely periodic palindromic 9-cycle 1, 6, 7, 4, 6, 4, 7, 6, 1. The units' digits of the a(n) form a purely periodic palindromic 4-cycle 1, 6, 6, 1. (End) Binomial transform of (1, 5, 5, 0, 0, 0, ...) and second partial sum of (1, 4, 5, 5, 5, ...). - Gary W. Adamson, Sep 09 2015 a(n) = a(-1-n) for all n in Z. - Michael Somos, Jan 25 2019 On the plane start with a single regular pentagon, and repeat the following procedure, "For each edge of any pentagon not already connected to an existing pentagon create a mirror image such that the mirror image does not overlap with an existing pentagon." a(n) is the number of pentagons occupying the plane after n repetitions. - Torlach Rush, Sep 14 2022 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). B. K. Teo and N. J. A. Sloane, Magic numbers in polygonal and polyhedral clusters, Inorgan. Chem. 24 (1985), 4545-4558. LINKS T. D. Noe, Table of n, a(n) for n = 0..1000 Paul Barry, Centered polygon numbers, heptagons and nonagons, and the Robbins numbers, arXiv:2104.01644 [math.CO], 2021. Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992; arXiv:0911.4975 [math.NT], 2009. Simon Plouffe, 1031 Generating Functions, Appendix to Thesis, Montreal, 1992 Cliff Reiter, Polygonal Numbers and Fifty One Stars, Lafayette College, Easton, PA (2019). Eric Weisstein's World of Mathematics, Centered Pentagonal Number. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA G.f.: (1 + 3*x + x^2)/(1 - x)^3. Simon Plouffe in his 1992 dissertation Narayana transform (A001263) of [1, 5, 0, 0, 0, ...]. - Gary W. Adamson, Dec 29 2007 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3), a(0)=1, a(1)=6, a(2)=16. - Jaume Oliver Lafont, Dec 02 2008 a(n) = 5*A000217(n) + 1 = 5*T(n) + 1, for n = 0, 1, 2, 3, ... and where T(n) = n*(n+1)/2 = n-th triangular number. - Thomas M. Green, Nov 25 2009 a(n) = a(n-1) + 5*n, with a(0)=1. - Vincenzo Librandi, Nov 18 2010 a(n) = A028895(n) + 1. - Omar E. Pol, Oct 03 2011 a(n) = 2*a(n-1) - a(n-2) + 5. - Ant King, Jun 12 2012 Sum_{n>=0} 1/a(n) = 2*Pi /sqrt(15) *tanh(Pi/2*sqrt(3/5)) = 1.360613169863... - Ant King, Jun 15 2012 a(n) = A101321(5,n). - R. J. Mathar, Jul 28 2016 E.g.f.: (2 + 10*x + 5*x^2)*exp(x)/2. - Ilya Gutkovskiy, Jul 28 2016 From Amiram Eldar, Jun 20 2020: (Start) Sum_{n>=0} a(n)/n! = 17*e/2. Sum_{n>=0} (-1)^(n+1)*a(n)/n! = 3/(2*e). (End) EXAMPLE a(2)= 5*T(2) + 1 = 5*3 + 1 = 16, a(4) = 5*T(4) + 1 = 5*10 + 1 = 51. - Thomas M. Green, Nov 16 2009 MAPLE A005891 := proc(n)     1+5*n*(1+n)/2 ; end proc: seq(A005891(n), n=0..40) ; # R. J. Mathar, Oct 07 2021 MATHEMATICA FoldList[#1 + #2 &, 1, 5 Range@ 40] (* Robert G. Wilson v, Feb 02 2011 *) LinearRecurrence[{3, -3, 1}, {1, 6, 16}, 50] (* Harvey P. Dale, Sep 08 2018 *) PROG (PARI) a(n)=5*n*(n+1)/2+1 \\ Charles R Greathouse IV, Mar 22 2016 (Magma) [5*n*(n+1)/2 + 1: n in [0..50]]; // G. C. Greubel, Nov 04 2017 CROSSREFS Cf. A028895, A001844, A003215, A004068 (partial sums), A006322, A001263. Partial sums of A008706. Equals second row of A167546 divided by 2. Sequence in context: A102214 A301679 A115007 * A108182 A244242 A092286 Adjacent sequences:  A005888 A005889 A005890 * A005892 A005893 A005894 KEYWORD nonn,easy,changed AUTHOR EXTENSIONS Formula corrected and relocated by Johannes W. Meijer, Nov 07 2009 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.

Last modified October 5 14:35 EDT 2022. Contains 357258 sequences. (Running on oeis4.)