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A024787 Number of 3's in all partitions of n. 14
0, 0, 1, 1, 2, 4, 6, 9, 15, 21, 31, 45, 63, 87, 122, 164, 222, 298, 395, 519, 683, 885, 1146, 1475, 1887, 2401, 3050, 3845, 4837, 6060, 7563, 9402, 11664, 14405, 17751, 21807, 26715, 32634, 39784, 48352, 58649, 70969, 85690, 103232, 124143, 148951, 178407, 213277, 254509 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

COMMENTS

Starting with the first 1 = row sums of triangle A173239. - Gary W. Adamson, Feb 13 2010

The sums of three successive terms give A000070. - Omar E. Pol, Jul 12 2012

a(n) is also the difference between the sum of 3rd largest and the sum of 4th largest elements in all partitions of n. - Omar E. Pol, Oct 25 2012

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..1000

Eric Weisstein's World of Mathematics, q-Pochhammer Symbol

FORMULA

a(n) = A181187(n,3) - A181187(n,4). - Omar E. Pol, Oct 25 2012

a(n) = Sum_{k=1..floor(n/3)} A263232(n,k). - Alois P. Heinz, Nov 01 2015

a(n) ~ exp(Pi*sqrt(2*n/3)) / (6*Pi*sqrt(2*n)) * (1 - 37*Pi/(24*sqrt(6*n)) + (37/48 + 937*Pi^2/6912)/n). - Vaclav Kotesovec, Nov 05 2016

G.f.: x^2/((1 - x^3)*(x)_inf), where (q)_inf is the q-Pochhammer symbol (the Euler function). - Vladimir Reshetnikov, Nov 22 2016

EXAMPLE

From Omar E. Pol, Oct 25 2012 (Start):

For n = 7 we have:

--------------------------------------

.                             Number

Partitions of 7               of 3's

--------------------------------------

7 .............................. 0

4 + 3 .......................... 1

5 + 2 .......................... 0

3 + 2 + 2 ...................... 1

6 + 1 .......................... 0

3 + 3 + 1 ...................... 2

4 + 2 + 1 ...................... 0

2 + 2 + 2 + 1 .................. 0

5 + 1 + 1 ...................... 0

3 + 2 + 1 + 1 .................. 1

4 + 1 + 1 + 1 .................. 0

2 + 2 + 1 + 1 + 1 .............. 0

3 + 1 + 1 + 1 + 1 .............. 1

2 + 1 + 1 + 1 + 1 + 1 .......... 0

1 + 1 + 1 + 1 + 1 + 1 + 1 ...... 0

------------------------------------

.      13 - 7 =                  6

The difference between the sum of the third column and the sum of the fourth column of the set of partitions of 7 is 13 - 7 = 6 and equals the number of 3's in all partitions of 7, so a(7) = 6.

(End)

MAPLE

b:= proc(n, i) option remember; local g;

      if n=0 or i=1 then [1, 0]

    else g:= `if`(i>n, [0$2], b(n-i, i));

         b(n, i-1) +g +[0, `if`(i=3, g[1], 0)]

      fi

    end:

a:= n-> b(n, n)[2]:

seq(a(n), n=1..100);  # Alois P. Heinz, Oct 27 2012

MATHEMATICA

Table[ Count[ Flatten[ IntegerPartitions[n]], 3], {n, 1, 50} ]

b[n_, i_] := b[n, i] = Module[{g}, If[n==0 || i==1, {1, 0}, g = If[i>n, {0, 0}, b[n-i, i]]; b[n, i-1] + g + {0, If[i==3, g[[1]], 0]}]]; a[n_] := b[n, n][[2]]; Table[a[n], {n, 1, 100}] (* Jean-Fran├žois Alcover, Oct 09 2015, after Alois P. Heinz *)

Join[{0, 0}, (1/((1 - x^3) QPochhammer[x]) + O[x]^50)[[3]]] (* Vladimir Reshetnikov, Nov 22 2016 *)

CROSSREFS

Cf. A066633, A024786, A024788, A024789, A024790, A024791, A024792, A024793, A024794, A263232.

Cf. A173239. - Gary W. Adamson, Feb 13 2010

Sequence in context: A127740 A323432 A327046 * A266648 A076922 A157679

Adjacent sequences:  A024784 A024785 A024786 * A024788 A024789 A024790

KEYWORD

nonn

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified October 17 06:08 EDT 2019. Contains 328106 sequences. (Running on oeis4.)