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 A130883 a(n) = 2*n^2 - n + 1. 29
 1, 2, 7, 16, 29, 46, 67, 92, 121, 154, 191, 232, 277, 326, 379, 436, 497, 562, 631, 704, 781, 862, 947, 1036, 1129, 1226, 1327, 1432, 1541, 1654, 1771, 1892, 2017, 2146, 2279, 2416, 2557, 2702, 2851, 3004, 3161, 3322, 3487, 3656, 3829 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Maximum number of regions determined by n bent lines (angular sectors). See GKP Reference. a(n)*Pi is the total length of half circle spiral after n rotations. It is formed as irregular spiral with two center points. At the 2nd stage, there are two alternatives: (1) select 2nd half circle radius, r2  = 2, the sequence will be A014105 or (2) select r2 = 0, the sequence will be A130883. See illustration in links. - Kival Ngaokrajang, Jan 19 2014 A128218(a(n)) = 2*n+1 and A128218(m) != 2*n+1 for m < a(n). - Reinhard Zumkeller, Jun 20 2015 REFERENCES R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics, 2nd ed., Addison-Wesley, Reading, MA, 1994, pp7-8. LINKS G. C. Greubel, Table of n, a(n) for n = 0..5000 Dmitry Efimov, Hafnian of two-parameter matrices, arXiv:2101.09722 [math.CO], 2021. Guo-Niu Han, Enumeration of Standard Puzzles Guo-Niu Han, Enumeration of Standard Puzzles [Cached copy] Ângela Mestre and José Agapito, Square Matrices Generated by Sequences of Riordan Arrays, J. Int. Seq., Vol. 22 (2019), Article 19.8.4. Kival Ngaokrajang, Illustration of irregular spirals (center points: 1, 2) Franck Ramaharo, Statistics on some classes of knot shadows, arXiv:1802.07701 [math.CO], 2018. Franck Ramaharo, A generating polynomial for the pretzel knot, arXiv:1805.10680 [math.CO], 2018. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = a(n-1) + 4*n - 3 for n > 0, a(0)=1. - Vincenzo Librandi, Nov 23 2010 a(n) = A000124(2*n) - 2*n. - Geoffrey Critzer, Mar 30 2011 O.g.f.: (4*x^2-x+1)/(1-x)^3. - Geoffrey Critzer, Mar 30 2011 a(n) = 2*a(n-1) - a(n-2) + 4. - Eric Werley, Jun 27 2011 a(0)=1, a(1)=2, a(2)=7; for n > 2, a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). - Harvey P. Dale, Jul 20 2011 a(n) = A128918(2*n). - Reinhard Zumkeller, Oct 27 2013 a(n) = 1 + A000384(n). - Omar E. Pol, Apr 27 2017 E.g.f.: (2*x^2 + x + 1)*exp(x). - G. C. Greubel, Jul 14 2017 a(n) = A152947(2*n+1). - Franck Maminirina Ramaharo, Jan 10 2018 MATHEMATICA a[n_]:=2*n^2-n+1; (* or *) Array[ -#*(1-#*2)+1&, 5!, 0] (* Vladimir Joseph Stephan Orlovsky, Dec 21 2008 *) LinearRecurrence[{3, -3, 1}, {1, 2, 7}, 50] (* Harvey P. Dale, Jul 20 2011 *) PROG (Haskell) a130883 = a128918 . (* 2)  -- Reinhard Zumkeller, Oct 27 2013 (PARI) a(n)=2*n^2-n+1 \\ Charles R Greathouse IV, Sep 24 2015 (Magma) [2*n^2 - n + 1 : n in [0..50]]; // Wesley Ivan Hurt, Mar 25 2020 (Python) def A130883(n): return n*(2*n - 1) + 1 # Chai Wah Wu, May 24 2022 CROSSREFS Cf. A000124, A084849, A128218, A128918. Cf. A270109. [Bruno Berselli, Mar 17 2016] Sequence in context: A293410 A348270 A162420 * A005581 A064468 A225311 Adjacent sequences:  A130880 A130881 A130882 * A130884 A130885 A130886 KEYWORD nonn,easy AUTHOR Mohammad K. Azarian, Jul 26 2007 STATUS approved

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Last modified October 6 09:29 EDT 2022. Contains 357263 sequences. (Running on oeis4.)