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A034804 Consider the sequence of 4-tuples {0,a,b,c} (c>=a+b; a,b,c>0)) which have the smallest integer 'c' required to reach {k,k,k,k} in n steps under map {r,s,t,u}->{|r-s|,|s-t|,|t-u|,|u-r|}. This sequence gives the third term 'b' of these quadruples. 1
0, 1, 0, 2, 1, 2, 4, 5, 6, 14, 17, 20, 48, 57, 68, 162, 193, 230, 548, 653, 778, 1854, 2209, 2632, 6272, 7473, 8904, 21218, 25281, 30122, 71780, 85525, 101902, 242830, 289329, 344732, 821488, 978793, 1166220, 2779074, 3311233, 3945294, 9401540 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

Table of n, a(n) for n=1..43.

FORMULA

a(n)= 2*Trib(2*q) if r=0; Trib(2*q-1)+Trib(2*q+1) if r=1; Trib(2*q)+Trib(2*q+1) if r=2 where q=[(n-1)/3], r=n-1 (mod 3) and Trib denotes the tribonacci sequence (A000073) with Trib(-1)=1. G.f.: (-x^7+2*x^6-2*x^5+2*x^4-2*x^3-x)/(x^9+x^6+3*x^3-1). Recurrence: a(n)=3*a(n-3)+a(n-6)+a(n-9), n >= 10.

EXAMPLE

a(10)=14 because {0, 5, 14, 31}->{5, 9, 17, 31}->{4, 8, 14, 26}->{4, 6, 12, 22}->{2, 6, 10, 18}->{4, 4, 8, 16}->{0, 4, 8, 12}->{4, 4, 4, 12}->{0, 0, 8, 8}->{0, 8, 0, 8}->{8, 8, 8, 8} ('b'=14 in the first 4-tuple and there is no quadruple with a+b<=c<=31 and 10 steps).

CROSSREFS

A034803, A045794 (or A065678) give the terms 'a' and 'c' respectively.

Sequence in context: A301364 A057061 A245660 * A050042 A166578 A138256

Adjacent sequences:  A034801 A034802 A034803 * A034805 A034806 A034807

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

EXTENSIONS

Better description, more terms, formula, etc. from Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), Jun 24 2001

STATUS

approved

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Last modified October 22 21:09 EDT 2018. Contains 316505 sequences. (Running on oeis4.)