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A330889 a(n) is the total number of parts in all partitions of n into consecutive parts that differ by 3. 12

%I #51 Dec 04 2020 11:55:11

%S 1,1,1,1,3,1,3,1,3,1,3,4,3,1,6,1,3,4,3,1,6,5,3,4,3,5,6,1,3,8,3,1,6,5,

%T 8,4,3,5,6,6,3,8,3,1,11,5,3,4,3,10,12,1,3,8,8,1,12,5,3,9,3,5,12,1,8,8,

%U 3,1,12,17,3,4,3,5,17,1,10,8,3,6,12,5,3,11,8,5,12,1,3,13

%N a(n) is the total number of parts in all partitions of n into consecutive parts that differ by 3.

%C The one-part partition n = n is included in the count.

%C For the relation to pentagonal numbers see also A330888.

%H Seiichi Manyama, <a href="/A330889/b330889.txt">Table of n, a(n) for n = 1..10000</a>

%F Conjecture: G.f.: Sum_{n>=1} n*x^(n*(3*n-1)/2)/(1-x^n).

%F Proof from _Matthew C. Russell_, Nov 21 2020:

%F The summation variable n runs over the number of parts in the partition.

%F For fixed n, the smallest such partition is:

%F 1 + 4 + 7 + ... + (3n-2).

%F The above sum is equal to n * (3*n-1) / 2. That's where the x^(n*(3*n-1)/2) factor comes from.

%F Then we want to (add 1 to every part), (add 2 to every part), etc. to get 2 + 5 + 8 + ..., 3 + 6 + 9 + ..., which corresponds to adding n, 2*n, 3*n, etc. to the base partition. So we divide by (1 - x^n).

%F Multiply by n (to count the total number of parts) and we are done. QED

%e For n = 21 there are three partitions of 21 into consecutive parts that differ by 3, including 21 as a partition. They are [21], [12, 9] and [10, 7, 4]. The number of parts of these partitions are 1, 2 and 3 respectively. The total number of parts is 1 + 2 + 3 = 6, so a(27) = 6.

%p A330889 := proc(n)

%p local a;

%p a := 0 ;

%p for k from 1 do

%p if n>= A000325(k) then

%p a := a+A330888(n,k);

%p else

%p return a;

%p end if;

%p end do:

%p end proc: # _R. J. Mathar_, Oct 02 2020

%t nmax = 100;

%t CoefficientList[Sum[n x^(n(3n-1)/2-1)/(1-x^n), {n, 1, nmax}]+O[x]^nmax, x] (* _Jean-François Alcover_, Nov 30 2020 *)

%o (PARI) my(N=66, x='x+O('x^N)); Vec(sum(k=1, N, k*x^(k*(3*k-1)/2)/(1-x^k))) \\ _Seiichi Manyama_, Dec 04 2020

%Y Row sums of A330888.

%Y Column k=3 of A334466.

%Y Sequences of the same family whose consecutive parts differs by k are: A000203 (k=0), A204217 (k=1), A066839 (k=2), this sequence (k=3), A334464 (k=4), A334732 (k=5), A334949 (k=6).

%Y Cf. A338730.

%K nonn,easy

%O 1,5

%A _Omar E. Pol_, Apr 30 2020

%E More terms from _R. J. Mathar_, Oct 02 2020

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Last modified March 29 06:57 EDT 2024. Contains 371265 sequences. (Running on oeis4.)