

A001399


a(n) is the number of partitions of n into at most 3 parts; also partitions of n+3 in which the greatest part is 3; also number of unlabeled multigraphs with 3 nodes and n edges.
(Formerly M0518 N0186)


191



1, 1, 2, 3, 4, 5, 7, 8, 10, 12, 14, 16, 19, 21, 24, 27, 30, 33, 37, 40, 44, 48, 52, 56, 61, 65, 70, 75, 80, 85, 91, 96, 102, 108, 114, 120, 127, 133, 140, 147, 154, 161, 169, 176, 184, 192, 200, 208, 217, 225, 234, 243, 252, 261, 271, 280, 290, 300, 310, 320, 331, 341
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OFFSET

0,3


COMMENTS

Also number of tripods (trees with exactly 3 leaves) on n vertices.  Eric W. Weisstein, Mar 05 2011
Also number of partitions of n+3 into exactly 3 parts; number of partitions of n in which the greatest part is less than or equal to 3; and the number of nonnegative solutions to b + 2c + 3d = n.
Also a(n) gives number of partitions of n+6 into 3 distinct parts and number of partitions of 2n+9 into 3 distinct and odd parts, e.g., 15 = 11 + 3 + 1 = 9 + 5 + 1 = 7 + 5 + 3.  Jon Perry, Jan 07 2004
Also bracelets with n+3 beads 3 of which are red (so there are 2 possibilities with 5 beads).
More generally, the number of partitions of n into at most k parts is also the number of partitions of n+k into k positive parts, the number of partitions of n+k in which the greatest part is k, the number of partitions of n in which the greatest part is less than or equal to k, the number of partitions of n+k(k+1)/2 into exactly k distinct positive parts, the number of nonnegative solutions to b + 2c + 3d + ... + kz = n and the number of nonnegative solutions to 2c + 3d + ... + kz <= n.  Henry Bottomley, Apr 17 2001
Also coefficient of q^n in the expansion of (m choose 3)_q as m goes to infinity.  Y. Kelly Itakura (yitkr(AT)mta.ca), Aug 21 2002
From Winston C. Yang (winston(AT)cs.wisc.edu), Apr 30 2002: (Start)
Write 1,2,3,4,... in a hexagonal spiral around 0, then a(n) for n > 0 is formed by the folding points (including the initial 1). The spiral begins:
.
858483828180
/ \
86 5655545352 79
/ / \ \
87 57 33323130 51 78
/ / / \ \ \
88 58 34 161514 29 50 77
/ / / / \ \ \ \
89 59 35 17 54 13 28 49 76
/ / / / / \ \ \ \ \
90 60 36 18 6 0 3 12 27 48 75
/ / / / / / / / / / /
91 61 37 19 7 12 11 26 47 74
\ \ \ \ / / / /
62 38 20 8910 25 46 73
\ \ \ / / /
63 39 21222324 45 72
\ \ / /
64 4041424344 71
\ /
656667686970
.
a(p) is maximal number of hexagons in a polyhex with perimeter at most 2p + 6. (End)
a(n3) is the number of partitions of n into 3 distinct parts, where 0 is allowed as a part. E.g., at n=9, we can write 8+1+0, 7+2+0, 6+3+0, 4+5+0, 1+2+6, 1+3+5 and 2+3+4, which is a(6)=7.  Jon Perry, Jul 08 2003
a(n) gives number of partitions of n+6 into parts <=3 where each part is used at least once (subtract 6=1+2+3 from n).  Jon Perry, Jul 03 2004
This is also the number of partitions of n+3 into exactly 3 parts (there is a 1to1 correspondence between the number of partitions of n+3 in which the greatest part is 3 and the number of partitions of n+3 into exactly three parts).  Graeme McRae, Feb 07 2005
Apply the Riordan array (1/(1x^3),x) to floor((n+2)/2).  Paul Barry, Apr 16 2005
Also, number of triangles that can be created with odd perimeter 3,5,7,9,11,... with all sides whole numbers. Note that triangles with even perimeter can be generated from the odd ones by increasing each side by 1. E.g., a(1) = 1 because perimeter 3 can make {1,1,1} 1 triangle. a(4) = 3 because perimeter 9 can make {1,4,4} {2,3,4} {3,3,3} 3 possible triangles.  Bruce Love (bruce_love(AT)ofs.edu.sg), Nov 20 2006
Also number of nonnegative solutions of the Diophantine equation x+2*y+3*z=n, cf. Pólya/Szegő reference.
Also a(n3), n >= 3, is the number of nonequivalent necklaces of 3 beads each of them painted by one of n colors.
The sequence {a(n3), n >= 3} solves the socalled Reis problem about convex kgons in case k=3 (see our comment to A032279).
a(n3) (n >= 3) is an essentially unimprovable upper estimate for the number of distinct values of the permanent in (0,1)circulants of order n with three 1's in every row. (End)
A001399(n) is the number of 3tuples (w,x,y) having all terms in {0,...,n} and w = 2*x+3*y.  Clark Kimberling, Jun 04 2012
Also, for n >= 3, a(n3) is the number of the distinct triangles in an ngon, see the Ngaokrajang links.  Kival Ngaokrajang, Mar 16 2013
Also, a(n) is the total number of 5curve coin patterns (5C4S type: 5 curves covering full 4 coins and symmetry) packing into fountain of coins base (n+3). See illustration in links.  Kival Ngaokrajang, Oct 16 2013
Also a(n) = half the number of minimal zero sequences for Z_n of length 3 [Ponomarenko].  N. J. A. Sloane, Feb 25 2014
Also, a(n) equals the number of linearlyindependent terms at 2nth order in the power series expansion of an Octahedral Rotational Energy Surface (cf. Harter & Patterson).  Bradley Klee, Jul 31 2015
Also Molien series for invariants of finite Coxeter groups D_3 and A_3.  N. J. A. Sloane, Jan 10 2016
Number of different distributions of n+6 identical balls in 3 boxes as x,y,z where 0 < x < y < z.  Ece Uslu and Esin Becenen, Jan 11 2016
a(n) is also the number of partitions of 2*n with <= n parts and no part >= 4. The bijection to partitions of n with no part >= 4 is: 1 <> 2, 2 <> 1 + 3, 3 <> 3 + 3 (observing the order of these rules). The < direction uses the following fact for partitions of 2*n with <= n parts and no part >=4: for each part 1 there is a part 3, and an even number (including 0) of remaining parts 3.  Wolfdieter Lang, May 21 2019


REFERENCES

R. Ayoub, An Introduction to the Analytic Theory of Numbers, Amer. Math. Soc., 1963; Chapter III, Problem 33.
L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 110, D(n); page 263, #18, P_n^{3}.
J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 517.
H. Gupta et al., Tables of Partitions. Royal Society Mathematical Tables, Vol. 4, Cambridge Univ. Press, 1958, p. 2.
F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 88, (4.1.18).
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, p. 275.
R. Honsberger, Mathematical Gems III, Math. Assoc. Amer., 1985, p. 39.
J. H. van Lint, Combinatorial Seminar Eindhoven, Lecture Notes Math., 382 (1974), see pp. 3334.
G. Pólya and G. Szegő, Problems and Theorems in Analysis I (Springer 1924, reprinted 1972), Part One, Chap. 1, Sect. 1, Problem 25.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

S. J. Cyvin, B. N. Cyvin, J. Brunvoll, I. Gutman, Chen Rongsi, S. ElBasil, and Zhang Fuji, Polygonal Systems Including the Corannulene and Coronene Homologs: Novel Applications of Pólya's Theorem, Z. Naturforsch., 52a (1997), 867873.
Eric Weisstein's World of Mathematics, Tripod.


FORMULA

G.f.: 1/((1  x) * (1  x^2) * (1  x^3)).
a(n) = round((n + 3)^2/12). Note that this cannot be of the form (2*i + 1)/2, so ties never arise.
a(n) = 1 + a(n2) + a(n3)  a(n5) for all n in Z.  Michael Somos, Sep 04 2006
P(n, 3) = (1/72) * (6*n^2  7  9*pcr{1, 1}(2, n) + 8*pcr{2, 1, 1}(3, n)) (see Comtet). [Here "pcr" stands for "prime circulator" and it is defined on p. 109 of Comtet, while the formula appears on p. 110.  Petros Hadjicostas, Oct 03 2019]
Let m > 0 and 3 <= p <= 2 be defined by n = 6*m+p3; then for n > 3, a(n) = 3*m^2 + p*m, and for n = 3, a(n) = 3*m^2 + p*m + 1.  Floor van Lamoen, Jul 23 2001
a(n) = 6*t(floor(n/6)) + (n%6) * (floor(n/6) + 1) + (n mod 6 == 0?1:0), where t(n) = n*(n+1)/2.
a(n) = ceiling(1/12*n^2 + 1/2*n) + (n mod 6 == 0?1:0).
[Here "n%6" means "n mod 6" while "(n mod 6 == 0?1:0)" means "if n mod 6 == 0 then 1, else 0" (as in C).]
(End)
a(n) = Sum_{i=0..floor(n/3)} 1 + floor((n  3*i)/2).  Jon Perry, Jun 27 2003
a(n) = Sum_{k=0..n} floor((k + 2)/2) * (cos(2*Pi*(n  k)/3 + Pi/3)/3 + sqrt(3) * sin(2*Pi*(nk)/3 + Pi/3)/3 + 1/3).  Paul Barry, Apr 16 2005
(m choose 3)_q = (q^m1) * (q^(m1)  1) * (q^(m2)  1)/((q^3  1) * (q^2  1) * (q  1)).
a(n) = Sum_{k=0..floor(n/2)} floor((3 + n  2*k)/3).  Paul Barry, Nov 11 2003
a(n) = 3 * Sum_{i=2..n+1} floor(i/2)  floor(i/3).  Thomas Wieder, Feb 11 2007
Identical to the number of points inside or on the boundary of the integer grid of {I, J}, bounded by the three straight lines I = 0, I  J = 0 and I + 2J = n.  Jonathan Vos Post, Jul 03 2007
Euler transform of length 3 sequence [ 1, 1, 1].  Michael Somos, Feb 25 2012
Let p(n, 3) be the number of 3part integer partitions in which every part is > 0.
Then for n >= 3, p(n, 3) is equal to:
(n^2  1)/12 when n is odd and 3 does not divide n.
(n^2 + 3)/12 when n is odd and 3 divides n.
(n^2  4)/12 when n is even and 3 does not divide n.
(n^2)/12 when n is even and 3 divides n.
For n >= 3, p(n, 3) = a(n3). (End)
Sum_{n>=0} 1/a(n) = 15/4  Pi/(2*sqrt(3)) + Pi^2/18 + tanh(Pi/(2*sqrt(3)))*Pi/sqrt(3).  Amiram Eldar, Sep 29 2022
E.g.f.: exp(x)*(9 + exp(2*x)*(47 + 42*x + 6*x^2) + 16*exp(x/2)*cos(sqrt(3)*x/2))/72.  Stefano Spezia, Mar 05 2023


EXAMPLE

G.f. = 1 + x + 2*x^2 + 3*x^3 + 4*x^4 + 5*x^5 + 7*x^6 + 8*x^7 + 10*x^8 + 12*x^9 + ...
Recall that in a necklace the adjacent beads have distinct colors. Suppose we have n colors with labels 1,...,n. Two colorings of the beads are equivalent if the cyclic sequences of the distances modulo n between labels of adjacent colors have the same period. If n=4, all colorings are equivalent. E.g., for the colorings {1,2,3} and {1,2,4} we have the same period {1,1,2} of distances modulo 4. So, a(n3)=a(1)=1. If n=5, then we have two such periods {1,1,3} and {1,2,2} modulo 5. Thus a(2)=2.  Vladimir Shevelev, Apr 23 2011
a(0) = 1, i.e., {1,2,3} Number of different distributions of 6 identical balls to 3 boxes as x,y and z where 0 < x < y < z.  Ece Uslu, Esin Becenen, Jan 11 2016
a(3) = 3, i.e., {1,2,6}, {1,3,5}, {2,3,4} Number of different distributions of 9 identical balls in 3 boxes as x,y and z where 0 < x < y < z.  Ece Uslu, Esin Becenen, Jan 11 2016
The a(0) = 1 through a(8) = 10 integer partitions of n with at most three parts are the following. The Heinz numbers of these partitions are given by A037144.
() (1) (2) (3) (4) (5) (6) (7) (8)
(11) (21) (22) (32) (33) (43) (44)
(111) (31) (41) (42) (52) (53)
(211) (221) (51) (61) (62)
(311) (222) (322) (71)
(321) (331) (332)
(411) (421) (422)
(511) (431)
(521)
(611)
The a(0) = 1 through a(7) = 8 integer partitions of n + 3 whose greatest part is 3 are the following. The Heinz numbers of these partitions are given by A080193.
(3) (31) (32) (33) (322) (332) (333) (3322)
(311) (321) (331) (3221) (3222) (3331)
(3111) (3211) (3311) (3321) (32221)
(31111) (32111) (32211) (33211)
(311111) (33111) (322111)
(321111) (331111)
(3111111) (3211111)
(31111111)
Nonisomorphic representatives of the a(0) = 1 through a(5) = 5 unlabeled multigraphs with 3 vertices and n edges are the following.
{} {12} {12,12} {12,12,12} {12,12,12,12} {12,12,12,12,12}
{13,23} {12,13,23} {12,13,23,23} {12,13,13,23,23}
{13,23,23} {13,13,23,23} {12,13,23,23,23}
{13,23,23,23} {13,13,23,23,23}
{13,23,23,23,23}
The a(0) = 1 through a(8) = 10 strict integer partitions of n  6 with three parts are the following (A = 10, B = 11). The Heinz numbers of these partitions are given by A007304.
(321) (421) (431) (432) (532) (542) (543) (643) (653)
(521) (531) (541) (632) (642) (652) (743)
(621) (631) (641) (651) (742) (752)
(721) (731) (732) (751) (761)
(821) (741) (832) (842)
(831) (841) (851)
(921) (931) (932)
(A21) (941)
(A31)
(B21)
The a(0) = 1 through a(8) = 10 integer partitions of n + 3 with three parts are the following. The Heinz numbers of these partitions are given by A014612.
(111) (211) (221) (222) (322) (332) (333) (433) (443)
(311) (321) (331) (422) (432) (442) (533)
(411) (421) (431) (441) (532) (542)
(511) (521) (522) (541) (551)
(611) (531) (622) (632)
(621) (631) (641)
(711) (721) (722)
(811) (731)
(821)
(911)
The a(0) = 1 through a(8) = 10 integer partitions of n whose greatest part is <= 3 are the following. The Heinz numbers of these partitions are given by A051037.
() (1) (2) (3) (22) (32) (33) (322) (332)
(11) (21) (31) (221) (222) (331) (2222)
(111) (211) (311) (321) (2221) (3221)
(1111) (2111) (2211) (3211) (3311)
(11111) (3111) (22111) (22211)
(21111) (31111) (32111)
(111111) (211111) (221111)
(1111111) (311111)
(2111111)
(11111111)
The a(0) = 1 through a(6) = 7 strict integer partitions of 2n+9 with 3 parts, all of which are odd, are the following. The Heinz numbers of these partitions are given by A307534.
(5,3,1) (7,3,1) (7,5,1) (7,5,3) (9,5,3) (9,7,3) (9,7,5)
(9,3,1) (9,5,1) (9,7,1) (11,5,3) (11,7,3)
(11,3,1) (11,5,1) (11,7,1) (11,9,1)
(13,3,1) (13,5,1) (13,5,3)
(15,3,1) (13,7,1)
(15,5,1)
(17,3,1)
The a(0) = 1 through a(8) = 10 strict integer partitions of n + 3 with 3 parts where 0 is allowed as a part (A = 10):
(210) (310) (320) (420) (430) (530) (540) (640) (650)
(410) (510) (520) (620) (630) (730) (740)
(321) (610) (710) (720) (820) (830)
(421) (431) (810) (910) (920)
(521) (432) (532) (A10)
(531) (541) (542)
(621) (631) (632)
(721) (641)
(731)
(821)
The a(0) = 1 through a(7) = 7 integer partitions of n + 6 whose distinct parts are 1, 2, and 3 are the following. The Heinz numbers of these partitions are given by A143207.
(321) (3211) (3221) (3321) (32221) (33221) (33321)
(32111) (32211) (33211) (322211) (322221)
(321111) (322111) (332111) (332211)
(3211111) (3221111) (3222111)
(32111111) (3321111)
(32211111)
(321111111)
(End)
Partitions of 2*n with <= n parts and no part >= 4: a(3) = 3 from (2^3), (1,2,3), (3^2) mapping to (1^3), (1,2), (3), the partitions of 3 with no part >= 4, respectively.  Wolfdieter Lang, May 21 2019


MAPLE

[ seq(1+floor((n^2+6*n)/12), n=0..60) ];
for n from 1 to 20 do result:=0: for i from 2 to n+1 do result:=result+(floor(i/2)floor(i/3)); od; result; od; # Thomas Wieder, Feb 11 2007
with(combstruct):ZL4:=[S, {S=Set(Cycle(Z, card<4))}, unlabeled]:seq(count(ZL4, size=n), n=0..61); # Zerinvary Lajos, Sep 24 2007
B:=[S, {S = Set(Sequence(Z, 1 <= card), card <=3)}, unlabelled]: seq(combstruct[count](B, size=n), n=0..61); # Zerinvary Lajos, Mar 21 2009


MATHEMATICA

CoefficientList[ Series[ 1/((1  x)*(1  x^2)*(1  x^3)), {x, 0, 65} ], x ]
Table[ Length[ IntegerPartitions[n, 3]], {n, 0, 61} ] (* corrected by JeanFrançois Alcover, Aug 08 2012 *)
k = 3; Table[(Apply[Plus, Map[EulerPhi[ # ]Binomial[n/#, k/# ] &, Divisors[GCD[n, k]]]]/n + Binomial[If[OddQ[n], n  1, n  If[OddQ[k], 2, 0]]/2, If[OddQ[k], k  1, k]/2])/2, {n, k, 50}] (* Robert A. Russell, Sep 27 2004 *)
LinearRecurrence[{1, 1, 0, 1, 1, 1}, {1, 1, 2, 3, 4, 5}, 70] (* Harvey P. Dale, Jun 21 2012 *)
a[ n_] := With[{m = Abs[n + 3]  3}, Length[ IntegerPartitions[ m, 3]]]; (* Michael Somos, Dec 25 2014 *)
k=3 (* Number of red beads in bracelet problem *); CoefficientList[Series[(1/k Plus@@(EulerPhi[#] (1x^#)^((k/#))&/@Divisors[k])+(1+x)/(1x^2)^Floor[(k+2)/2])/2, {x, 0, 50}], x] (* Herbert Kociemba, Nov 04 2016 *)
Table[Length[Select[IntegerPartitions[n, {3}], UnsameQ@@#&]], {n, 0, 30}] (* Gus Wiseman, Apr 15 2019 *)


PROG

(PARI) {a(n) = round((n + 3)^2 / 12)}; /* Michael Somos, Sep 04 2006 */
(Haskell)
a001399 = p [1, 2, 3] where
p _ 0 = 1
p [] _ = 0
p ks'@(k:ks) m = if m < k then 0 else p ks' (m  k) + p ks m
(Magma) I:=[1, 1, 2, 3, 4, 5]; [n le 6 select I[n] else Self(n1)+Self(n2)Self(n4)Self(n5)+Self(n6): n in [1..80]]; // Vincenzo Librandi, Feb 14 2015
(Magma) [#RestrictedPartitions(n, {1, 2, 3}): n in [0..62]]; // Marius A. Burtea, Jan 06 2019


CROSSREFS

Cf. A008724, A003082, A117485, A026810, A026811, A026812, A026813, A026814, A026815, A026816, A000228, A036496, A008619, A001400, A001401, A069905, A008615, row 3 of A192517.


KEYWORD

nonn,easy,nice


AUTHOR



EXTENSIONS



STATUS

approved



