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A151961 Semiperimeter of the n-th Heronian triangle. 3
3, 6, 21, 78, 291, 1086, 4053, 15126, 56451, 210678, 786261, 2934366, 10951203, 40870446, 152530581, 569251878, 2124476931, 7928655846, 29590146453, 110431929966, 412137573411, 1538118363678, 5740335881301, 21423225161526, 79952564764803, 298387033897686 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The side lengths are consecutive integers (A016064) and the area is an integer (A011945).

Except for the first term, positive values of x (or y) satisfying  x^2-4xy+y^2+27 = 0. - Colin Barker, Feb 08 2014

Except for the first term, positive values of x (or y) satisfying x^2 - 14xy + y^2 + 432 = 0. - Colin Barker, Feb 16 2014

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..200

Index entries for linear recurrences with constant coefficients, signature (4,-1).

FORMULA

a(n) = 3*(A016064(n-1)+1)/2 = 3*A003500(n-1)/2. - R. J. Mathar, Jul 27 2009

a(n) = 4*a(n-1)-a(n-2). G.f.: 3*x*(1-2*x)/(1-4*x+x^2). - Colin Barker, Mar 30 2012

a(n) = (3*(2-sqrt(3))^n*(2+sqrt(3))-3*(-2+sqrt(3))*(2+sqrt(3))^n)/2. - Colin Barker, Oct 12 2015

MATHEMATICA

CoefficientList[Series[3 (1 - 2 x)/(1 - 4 x + x^2), {x, 0, 30}], x] (* Vincenzo Librandi, Feb 11 2014 *)

PROG

(MAGMA) I:=[3, 6]; [n le 2 select I[n] else 4*Self(n-1)-Self(n-2): n in [1..30]]; // Vincenzo Librandi, Feb 11 2014

(PARI) Vec(3*x*(1-2*x)/(1-4*x+x^2) + O(x^40)) \\ Colin Barker, Oct 12 2015

CROSSREFS

Cf. A003500, A011945, A016064.

Sequence in context: A261582 A135686 A218244 * A025229 A073951 A032098

Adjacent sequences:  A151958 A151959 A151960 * A151962 A151963 A151964

KEYWORD

nonn,easy

AUTHOR

Juri-Stepan Gerasimov, Jul 13 2009

EXTENSIONS

More terms from R. J. Mathar, Jul 27 2009

STATUS

approved

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Last modified September 18 09:59 EDT 2019. Contains 327170 sequences. (Running on oeis4.)