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A056107 Third spoke of a hexagonal spiral. 36
1, 4, 13, 28, 49, 76, 109, 148, 193, 244, 301, 364, 433, 508, 589, 676, 769, 868, 973, 1084, 1201, 1324, 1453, 1588, 1729, 1876, 2029, 2188, 2353, 2524, 2701, 2884, 3073, 3268, 3469, 3676, 3889, 4108, 4333, 4564, 4801, 5044, 5293, 5548, 5809, 6076, 6349 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

a(n+1) is the number of lines crossing n cells of an n X n X n cube. - Lekraj Beedassy, Jul 29 2005

Equals binomial transform of [1, 3, 6, 0, 0, 0,...]. - Gary W. Adamson, May 03 2008

Each term a(n), with n>1 represents the area of the right trapezoid with bases whose values are equal to hex number A003215(n) and A003215(n+1)and height equal to 1. The right trapezoid is formed by a rectangle with the sides equal to A003215(n) and 1 and a right triangle whose area is 3*n with the greater cathetus equal to the difference A003215(n+1)-A003215(n). - Giacomo Fecondo, Jun 11 2010

2*a(n)^2 is of the form x^4+y^4+(x+y)^4. In fact, 2*a(n)^2 = (n-1)^4+(n+1)^4+(2n)^4. - Bruno Berselli, Jul 16 2013

Numbers m such that m+(m-1)+(m-2) is a square. - César Aguilera, May 26 2015

After 4, twice each term belongs to A181123: 2*a(n) = (n+1)^3 - (n-1)^3. - Bruno Berselli, Mar 09 2016

This is a subsequence of A003136: a(n) = (n-1)^2 + (n-1)*(n+1) + (n+1)^2. - Bruno Berselli, Feb 08 2017

For n > 3, also the number of (not necessarily maximum) cliques in the n X n torus grid graph. - Eric W. Weisstein, Nov 30 2017

REFERENCES

E. J. Barbeau et al., Five Hundred Mathematical Challenges, Problem 444 pp. 42;195 MAA Washington DC 1995.

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 0..5000

H. Bottomley, Illustration of initial terms

A. J. C. Cunningham, Factorisation of N and N' = (x^n -+ y^n) / (x -+ y [when x-y=n], Messenger Math., 54 (1924), 17-21 [Incomplete annotated scanned copy]

G. Nebe and N. J. A. Sloane, Home page for hexagonal (or triangular) lattice A2

A. L. Rubinoff and Leo Moser, Solution to Problem E773, The American Mathematical Monthly, Vol. 55, No. 2 (Feb., 1948), p. 99.

Eric Weisstein's World of Mathematics, Clique

Eric Weisstein's World of Mathematics, Torus Grid Graph

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

FORMULA

a(n) = 3*n^2 + 1.

a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n>2.

G.f.: (1+x+4*x^2)/(1-x)^3.

a(n) = a(n-1) + 6*n - 3 for n>0.

a(n) = 2*a(n-1) - a(n-2) + 6 for n>1.

a(n) = A056105(n) + 2*n = A056106(n) + n.

a(n) = A056108(n) - n = A056109(n) - 2*n = A003215(n) - 3*n.

a(n) = (A000578(n+1) - A000578(n-1))/2. - Lekraj Beedassy, Jul 29 2005

a(n) = A132111(n+1,n-1) for n>1. - Reinhard Zumkeller, Aug 10 2007

E.g.f.: (1 +3*x +3*x^2)*exp(x). - G. C. Greubel, Dec 02 2018

MAPLE

seq(3*n^2+1, n=0..46); # Nathaniel Johnston, Jun 26 2011

MATHEMATICA

Table[3 n^2 + 1, {n, 100}] (* Vladimir Joseph Stephan Orlovsky, Jun 26 2011 *)

LinearRecurrence[{3, -3, 1}, {1, 4, 13}, 47] (* Michael De Vlieger, Feb 08 2017 *)

CoefficientList[Series[(1 + x + 4 x^2)/(1 - x)^3, {x, 0, 46}], x] (* Michael De Vlieger, Feb 08 2017 *)

1 + 3 Range[0, 20]^2 (* Eric W. Weisstein, Nov 30 2017 *)

PROG

(PARI) for(n=0, 1000, if(issquare(n+(n-1)+(n-2)), print1(n", "))); \\ César Aguilera, May 26 2015

(PARI) a(n) = 3*n^2 + 1; \\ Altug Alkan, Feb 08 2017

(MAGMA) [3*n^2 + 1: n in [0..40]]; // G. C. Greubel, Dec 02 2018

(Sage) [3*n^2 + 1 for n in range(40)] # G. C. Greubel, Dec 02 2018

(GAP) List([0..40], n -> 3*n^2 + 1); # G. C. Greubel, Dec 02 2018

CROSSREFS

Cf. A002648 for primes in this sequence, A054552 for example of square (or octagonal) spiral spoke.

Cf. A201053.

Sequence in context: A272455 A220745 A298017 * A155433 A272746 A273557

Adjacent sequences:  A056104 A056105 A056106 * A056108 A056109 A056110

KEYWORD

nonn,easy,changed

AUTHOR

Henry Bottomley, Jun 09 2000

STATUS

approved

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Last modified December 11 22:04 EST 2018. Contains 318052 sequences. (Running on oeis4.)