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A278989 a(n) is the number of words of length n over an alphabet of size 4 that are in standard order and which have the property that every letter that appears in the word is repeated. 1
0, 0, 1, 1, 4, 11, 41, 162, 715, 3425, 16777, 80928, 379347, 1726375, 7654817, 33219630, 141692075, 596122477, 2480969257, 10237751324, 41963944275, 171103765747, 694775280993, 2812004330666, 11352134320523, 45736973060601, 183981143571721, 739167464021912, 2966826380664595, 11899055223201855 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

LINKS

Table of n, a(n) for n=0..29.

Joerg Arndt and N. J. A. Sloane, Counting Words that are in "Standard Order"

FORMULA

Conjectures from Colin Barker, Nov 25 2017: (Start)

G.f.: x^2*(1 - 19*x + 159*x^2 - 776*x^3 + 2474*x^4 - 5498*x^5 + 8993*x^6 - 11471*x^7 + 11815*x^8 - 9478*x^9 + 5348*x^10 - 1848*x^11 + 288*x^12) / ((1 - x)^4*(1 - 2*x)^3*(1 - 3*x)^2*(1 - 4*x)).

a(n) = 20*a(n-1) - 175*a(n-2) + 882*a(n-3) - 2835*a(n-4) + 6072*a(n-5) - 8777*a(n-6) + 8458*a(n-7) - 5204*a(n-8) + 1848*a(n-9) - 288*a(n-10) for n > 14.

(End)

CROSSREFS

A row of the array in A278987.

Sequence in context: A278988 A214188 A214239 * A000296 A032265 A320155

Adjacent sequences:  A278986 A278987 A278988 * A278990 A278991 A278992

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Dec 06 2016

STATUS

approved

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Last modified May 27 21:52 EDT 2020. Contains 334671 sequences. (Running on oeis4.)