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A005269 a(n) = number of length-n sequences s with s[1]=1, s[2]=1, s[k-1] <=s[k] <= s[k-2]+s[k-1] (s is called a sub-Fibonacci sequence of length n).
(Formerly M1234)
8
1, 2, 4, 10, 31, 127, 711, 5621, 64049, 1067599, 26287664, 963023487, 52766766100, 4342736509018, 538755914902622, 101067429677072459, 28751803102222498512, 12436935036300286507123, 8200693250120852291693833, 8262592110164298068793701546 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

REFERENCES

Fishburn, Peter C.; Roberts, Fred S., Uniqueness in finite measurement. Applications of combinatorics and graph theory to the biological and social sciences, 103--137, IMA Vol. Math. Appl., 17, Springer, New York, 1989. MR1009374 (90e:92099)

Fishburn, Peter C.; Roberts, Fred S.; Elementary sequences, sub-Fibonacci sequences. Discrete Appl. Math. 44 (1993), no. 1-3, 261-281.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Alois P. Heinz, Table of n, a(n) for n = 2..70

Fishburn, Peter C.; Roberts, Fred S., Uniqueness in finite measurement, in Applications of combinatorics and graph theory to the biological and social sciences, 103--137, IMA Vol. Math. Appl., 17, Springer, New York, 1989. MR1009374 (90e:92099). [Annotated scan of five pages only]

FORMULA

See the Maple program; f[k](x, y) is the number of sequences s[1], s[2], ..., s[k+2] such that s[1]=x, s[2]=y, s[j-1] <=s[j] <= s[j-2]+s[j-1]. - Emeric Deutsch and Don Reble, Feb 07 2005

EXAMPLE

G.f. = x^2 + 2*x^3 + 4*x^4 + 10*x^5 + 31*x^6 + 127*x^7 + 711*x^8 + 5621*x^9 + ...

a(4)=4 because we have (1,1,1,1), (1,1,1,2), (1,1,2,2), (1,1,2,3).

MAPLE

f[0]:=1:for k from 0 to 19 do f[k+1]:=expand(sum(subs({x=y, y=z}, f[k]), z=y..x+y)) od: seq(subs({x=1, y=1}, f[k]), k=0..19);

PROG

(PARI) {a(n) = if(n<2, return(0)); my(c, e); forvec(s=vector(n, i, [1, fibonacci(i)]), e=0; for(k=3, n, if( s[k-1]>s[k] || s[k]>s[k-2]+s[k-1], e=1; break)); if(e, next); c++, 1); c}; /* Michael Somos, Dec 02 2016 */

CROSSREFS

Sequences in the Fishburn-Roberts (1989) article: A005269, A005268, A234595, A005272, A003513, A008926.

Sequence in context: A138415 A005268 A243931 * A070900 A296003 A263662

Adjacent sequences:  A005266 A005267 A005268 * A005270 A005271 A005272

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms from Emeric Deutsch and Don Reble, Feb 07 2005

STATUS

approved

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Last modified February 26 11:49 EST 2020. Contains 332279 sequences. (Running on oeis4.)