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A106710 Number of words with n letters from an alphabet of size 26 with at least two equal consecutive letters. 2
0, 26, 1326, 50726, 1725126, 55009526, 1684153926, 50135658326, 1462218522726, 41984966747126, 1190791264331526, 33440126095275926, 931432109043580326, 25766955599293244726, 708683864685628269126, 19394355959426432653526, 528467641885089690397926 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Colin Barker, Table of n, a(n) for n = 1..706

Index entries for linear recurrences with constant coefficients, signature (51,-650).

FORMULA

a(n) = 26^n - 26*25^(n - 1).

From Colin Barker, Nov 05 2015: (Start)

a(n) = 51*a(n-1) - 650*a(n-2) for n>2.

G.f.: 26*x^2 / ((25*x-1)*(26*x-1)). (End)

EXAMPLE

a(3) = 1326 because 26^3 - 26*(25^2) = 1326.

PROG

(PARI) a(n) = 26^n - 26*(25^(n - 1)); \\ Michel Marcus, Aug 14 2013

(PARI) concat(0, Vec(26*x^2/((25*x-1)*(26*x-1)) + O(x^100))) \\ Colin Barker, Nov 05 2015

CROSSREFS

Sequence in context: A187463 A160311 A220955 * A114052 A042303 A042300

Adjacent sequences:  A106707 A106708 A106709 * A106711 A106712 A106713

KEYWORD

nonn,easy

AUTHOR

Luca Colucci, May 14 2005

EXTENSIONS

More terms from Michel Marcus, Aug 14 2013

STATUS

approved

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Last modified August 25 04:05 EDT 2019. Contains 326318 sequences. (Running on oeis4.)