OFFSET
1,4
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..1000
FORMULA
G.f.: Sum_{k>=1} x^(3*k)/(1 - x^k) / Product_{j>=1} (1 - x^j). - Ilya Gutkovskiy, Mar 05 2021
EXAMPLE
a(4) = 2, because 2 parts have size >= 3 in all partitions of 4: [1,1,1,1], [1,1,2], [2,2], [1,3], [4].
MAPLE
b:= proc(n, i) option remember; local f, g;
if n=0 then [1, 0]
elif i<1 then [0, 0]
elif i>n then b(n, i-1)
else f:= b(n, i-1); g:= b(n-i, i);
[f[1]+g[1], f[2]+g[2] +`if`(i>2, g[1], 0)]
fi
end:
a:= n-> b(n, n)[2]:
seq(a(n), n=1..50); # Alois P. Heinz, Feb 19 2012
MATHEMATICA
b[n_, i_] := b[n, i] = Module[{f, g}, Which[n == 0, {1, 0}, i < 1, {0, 0}, i > n, b[n, i - 1], True, f = b[n, i - 1]; g = b[n - i, i]; {f[[1]] + g[[1]], f[[2]] + g[[2]] + If[i > 2, g[[1]], 0]}]];
a[n_] := b[n, n][[2]];
Array[a, 50] (* Jean-François Alcover, Nov 12 2020, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Feb 18 2012
EXTENSIONS
More terms from Alois P. Heinz, Feb 18 2012
STATUS
approved