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A000466 a(n) = 4*n^2 - 1. 43
-1, 3, 15, 35, 63, 99, 143, 195, 255, 323, 399, 483, 575, 675, 783, 899, 1023, 1155, 1295, 1443, 1599, 1763, 1935, 2115, 2303, 2499, 2703, 2915, 3135, 3363, 3599, 3843, 4095, 4355, 4623, 4899, 5183, 5475, 5775, 6083, 6399, 6723, 7055, 7395 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Sum_{n>=1} (-1)^n*a(n)/n! = 1 - 1/e = A068996. - Gerald McGarvey, Nov 06 2007

Sequence arises from reading the line from -1, in the direction -1, 15,... and the same line from 3, in the direction 3, 35,..., in the square spiral whose nonnegative vertices are the squares A000290. - Omar E. Pol, May 24 2008

a(n) is the product of the consecutive odd integers 2n-1 and 2n+1 (cf. A005408). - Doug Bell, Mar 08 2009

For n>0: a(n) = A176271(2*n,n); cf. A016754, A053755. - Reinhard Zumkeller, Apr 13 2010

a(n+1) gives the curvature c(n) of the n-th circle touching the two equal semicircles of the symmetric arbelos (1/2, 1/2) and the (n-1)-st circle, with input c(0) = 3  =  A059100(1) (referring to the second circle of the Pappus chain), for n >= 0. - Wolfdieter Lang and Kival Ngaokrajang, Jul 03 2015

After 3, a(n) is pseudoprime to base 2n. For example: (2*2)^(a(2)-1) == 1 (mod a(2)), in fact 4^14 = 15*17895697+1. - Bruno Berselli, Sep 24 2015

After -1, numbers m such that m+1 and m/4+1 are squares. - Bruno Berselli, Mar 03 2016

After -1, the least common multiple of 2*m+1 and 2*m-1. - Colin Barker, Feb 11 2017

REFERENCES

T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 3.

L. B. W. Jolley, Summation of Series, Dover, 2nd ed., 1961.

Granino A. Korn and Theresa M. Korn, Mathematical Handbook for Scientists and Engineers, McGraw-Hill Book Company, New York (1968), pp. 980-981.

A. Languasco and A. Zaccagnini, Manuale di Crittografia, Ulrico Hoepli Editore (2015), p. 259.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..900

M. Janjic and B. Petkovic, A Counting Function, arXiv 1301.4550 [math.CO], 2013.

Kival Ngaokrajang, Illustration of the Pappus chain (downwards direction)

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

FORMULA

G.f.: ( 1-6*x-3*x^2 ) / (x-1)^3 . - R. J. Mathar, Mar 24 2011

Sum_{n>=1} 1/a(n) = 1/2 [Jolley eq. 233]. - Benoit Cloitre, Apr 05 2002

Sum_{n>=1} 2/a(n) = 1 = 2/3 + 2/15 + 2/35 + 2/63 + 2/99 + 2/143, ..., with partial sums: 2/3, 4/5, 6/7, 8/9, 10/11, 12/13, 14/15, ... - Gary W. Adamson, Jun 16 2003

1/3 + Sum_{n>=2} 4/a(n) = 1 = 1/3 + 4/15 + 4/35 + 4/63, ..., with partial sums: 1/3, 3/5, 5/7, 7/9, 9/11, ..., (2n+1)/(2n+3). - Gary W. Adamson, Jun 18 2003

Sum_{n>=0} 2/a(2n+1) = Pi/4 = 2/3 + 2/35 + 2/99, ... = (1 - 1/3) + (1/5 - 2/7) + (1/9 - 1/11) + ... = Sum_{n>=0} (-1)^n/(2*n+1). - Gary W. Adamson, Jun 22 2003

Product(n>=1, (a(n)+1)/a(n)) = pi/2 (Wallis formula). - Mohammed Bouayoun (mohammed.bouayoun(AT)sanef.com), Mar 03 2004

a(n)+2 = A053755(n). - Zak Seidov, Jan 16 2007

a(n)^2 + A008586(n)^2 = A053755(n)^2 (Pythagorean triple). - Zak Seidov, Jan 16 2007

a(n) = a(n-1)+8*n-4 (with a(0)=-1). - Vincenzo Librandi, Dec 17 2010

sum_{n>=1} (-1)^(n+1)/a(n) =  Pi/4-1/2 = (A019669-1)/2. [Jolley eq (366)]. - R. J. Mathar, Mar 24 2011

For n>0 a(n) = 2/(Integral_{x=0..Pi/2} (sin(x))^3*(cos(x))^(2*n-2)). - Francesco Daddi, Aug 02 2011

Nonlinear recurrence for c(n) = a(n+1) (see the arbelos comment above) from Descartes' three circle theorem (see the links under A259555): c(n) = 4 + c(n-1) + 4*sqrt(c(n-1) + 1), with input c(0) = 3  =  A059100(1), for n >= 0. The appropriate solution of this recurrence is c(n-1) + 1 = 4*n^2. - Wolfdieter Lang, Jul 03 2015

a(n) = 3*Pochhammer(5/2,n-1)/Pochhammer(1/2,n-1). Hence, the e.g.f. for a(n+1), i.e., dropping the first term, is 3* 1F1(5/2;1/2;x), with 1F1 being the confluent hypergeometric function (also known as Kummer's). - Stanislav Sykora, May 26 2016

E.g.f.: (-1 + 4*x + 4*x^2)*exp(x). - Ilya Gutkovskiy, May 26 2016

MAPLE

A000466:=n->4*n^2-1; seq(A000466(n), n=0..100); # Wesley Ivan Hurt, Nov 19 2013

MATHEMATICA

4 Range[0, 50]^2-1 (* Harvey P. Dale, Jan 23 2011 *)

PROG

(MAGMA) [4*n^2-1: n in [0..50]]; // Vincenzo Librandi, Apr 26 2011

(PARI) a(n)=4*n^2-1 \\ Charles R Greathouse IV, Oct 27 2011

(Maxima) makelist(4*n^2-1, n, 0, 50); /* Martin Ettl, Nov 12 2012 */

(Sage) [4*n^2-1 for n in (0..50)] # Bruno Berselli, Sep 24 2015

CROSSREFS

Cf. A000290, A001539, A016286, A016742.

Factor of A160466. Superset of A037074.

Cf. A059100 (curvatures for a Pappus chain).

Sequence in context: A009261 A102790 A236693 * A241237 A145949 A015809

Adjacent sequences:  A000463 A000464 A000465 * A000467 A000468 A000469

KEYWORD

sign,easy

AUTHOR

Chan Siu Kee (skchan5(AT)hkein.ie.cuhk.hk)

STATUS

approved

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Last modified August 17 20:59 EDT 2017. Contains 290655 sequences.