OFFSET
1,2
COMMENTS
A partition of N>=1, given by its positive exponents e[1], e[2], ..., e[M], with largest part M and Sum_{j=1..M} j*e[j] = N, defines an m-multiset repetition class if it has m := Sum_{j=1..M} e[j] parts and the exponents are nonincreasing: e[1] >= e[2] >= ... >= e[M] >= 1.
See A176723 for the characteristic array for these partitions in Abramowitz-Stegun order.
The largest part M can be 1,2,...,Mmax(N), where Mmax(N)is the index of the largest triangular number smaller or equal to N. E.g., Mmax(7)= 3.
The minimal number of parts of these partitions of N is given by A185977 = [1, 2, 2, 3, 4, 3, 4, 5, 5, 4, 5, 6, 6, 7, 5, 6, 7, 7, 8, 8, 6, 7, 8, 8, 9, ...].
a(n) is the total number of such nonincreasing exponent sequences for N from 1 to n.
a(n) is also the total number of partitions of N, with 1<=N<=n, into nonzero triangular numbers A000217. See A007294.
a(n) is the partial sum of A007294 without the leading 1.
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{k=1..n}A007294(k), n>=1.
MAPLE
b:= proc(n, i) option remember;
if n<0 then 0
elif n=0 then 1
elif i=0 then 0
else b(n, i-1) +b(n-i*(i+1)/2, i)
fi
end:
a007294:= n-> b(n, floor(sqrt(2*n))): # Alois P. Heinz code for A007294
A007294:= [seq(a007294(n), n=1..100)]:
ListTools:-PartialSums(A007294); # Robert Israel, Apr 15 2016
MATHEMATICA
b[n_, i_] := b[n, i] = Which[n<0, 0, n==0, 1, i==0, 0, True, b[n, i-1] + b[n-i*(i+1)/2, i]]; Accumulate[Table[b[n, Floor[Sqrt[2n]]], {n, 1, 60}]] (* Jean-François Alcover, Feb 05 2017, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Mar 07 2011
EXTENSIONS
Changed by the author in response to comments by Franklin T. Adams-Watters, Apr 02 2011
STATUS
approved