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A167606 Number of compositions of n where each pair of adjacent parts is relatively prime. 60
1, 1, 2, 4, 7, 14, 25, 48, 90, 168, 316, 594, 1116, 2096, 3935, 7388, 13877, 26061, 48944, 91919, 172623, 324188, 608827, 1143390, 2147309, 4032677, 7573426, 14223008, 26711028, 50163722, 94208254, 176924559, 332267039, 624002605, 1171886500, 2200820905 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,3
LINKS
FORMULA
a(n) ~ c * d^n, where d=1.8780154065731862176678940156530410192010138618103068156064519919669849911..., c=0.5795813856338135589080831265343299561832275012313700387790334792220408848... - Vaclav Kotesovec, May 01 2014
EXAMPLE
For n = 4, there are 8 compositions: [4], [3,1], [2,2], [2,1,1], [1,3], [1,2,1], [1,1,2], and [1,1,1,1]. Of these, only [2,2] has adjacent terms that are not relatively prime, so a(4) = 7.
MAPLE
b:= proc(n, i) option remember; `if`(n=0, 1,
add(`if`(igcd(i, j)=1, b(n-j, j), 0), j=1..n))
end:
a:= n-> b(n, 1):
seq(a(n), n=0..40); # Alois P. Heinz, Apr 27 2014
MATHEMATICA
b[n_, i_] := b[n, i] = If[n==0, 1, Sum[If[GCD[i, j]==1, b[n-j, j], 0], {j, n}]];
a[n_] := b[n, 1];
a /@ Range[0, 40] (* Jean-François Alcover, Apr 25 2020, after Alois P. Heinz *)
PROG
(PARI) am(n)={local(r); r=matrix(n, n);
for(k=1, n,
for(i=1, k-1, r[k, i]=sum(j=1, k-i, if(gcd(i, j)==1, r[k-i, j], 0))); r[k, k]=1);
r}
al(n)=local(m); m=am(n); vector(n, k, sum(i=1, k, m[k, i]))
a(left, last=1)={local(r); if(left==0, return(1));
for(k=1, left, if(gcd(k, last)==1, r+=a(left-k, k))); r}
CROSSREFS
Sequence in context: A287185 A065491 A072810 * A065455 A220842 A026010
KEYWORD
nonn
AUTHOR
STATUS
approved

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Last modified June 28 12:12 EDT 2024. Contains 373786 sequences. (Running on oeis4.)