This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A266755 #86 Jun 03 2025 15:10:08
%S A266755 1,0,1,1,2,1,3,2,4,3,5,4,7,5,8,7,10,8,12,10,14,12,16,14,19,16,21,19,
%T A266755 24,21,27,24,30,27,33,30,37,33,40,37,44,40,48,44,52,48,56,52,61,56,65,
%U A266755 61,70,65,75,70,80,75,85,80,91,85,96,91,102,96,108,102,114,108,120,114,127,120,133,127,140,133,147,140,154,147,161,154,169
%N A266755 Expansion of 1/((1-x^2)*(1-x^3)*(1-x^4)).
%C A266755 This is the same as A005044 but without the three leading zeros. There are so many situations where one wants this sequence rather than A005044 that it seems appropriate for it to have its own entry.
%C A266755 But see A005044 (still the main entry) for numerous applications and references.
%C A266755 Also, Molien series for invariants of finite Coxeter group D_3.
%C A266755 The Molien series for the finite Coxeter group of type D_k (k >= 3) has g.f. = 1/Product_i (1-x^(1+m_i)) where the m_i are [1,3,5,...,2k-3,k-1]. If k is even only even powers of x appear, and we bisect the sequence.
%C A266755 Also, Molien series for invariants of finite Coxeter group A_3. The Molien series for the finite Coxeter group of type A_k (k >= 1) has g.f. = 1/Product_{i=2..k+1} (1-x^i). Note that this is the root system A_k not the alternating group Alt_k.
%C A266755 a(n) is the number of partitions of n into parts 2, 3, and 4. - _Joerg Arndt_, Apr 16 2017
%C A266755 From _Gus Wiseman_, May 23 2021: (Start)
%C A266755 Also the number of integer partitions of n into at most n/2 parts, none greater than 3. The case of any maximum is A110618. The case of any length is A001399. The Heinz numbers of these partitions are given by A344293.
%C A266755 For example, the a(2) = 1 through a(13) = 5 partitions are:
%C A266755 2 3 22 32 33 322 332 333 3322 3332 3333 33322
%C A266755 31 222 331 2222 3222 3331 32222 33222 33331
%C A266755 321 3221 3321 22222 33221 33321 322222
%C A266755 3311 32221 33311 222222 332221
%C A266755 33211 322221 333211
%C A266755 332211
%C A266755 333111
%C A266755 (End)
%D A266755 J. E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge, 1990. See Table 3.1, page 59.
%H A266755 G. C. Greubel, <a href="/A266755/b266755.txt">Table of n, a(n) for n = 0..1000</a>
%H A266755 Sara C. Billey, Matjaž Konvalinka, and Joshua P. Swanson, <a href="https://arxiv.org/abs/1905.00975">Asymptotic normality of the major index on standard tableaux</a>, arXiv:1905.00975 [math.CO], 2019.
%H A266755 <a href="/index/Mo#Molien">Index entries for Molien series</a>
%H A266755 <a href="/index/Rec#order_09">Index entries for linear recurrences with constant coefficients</a>, signature (0,1,1,1,-1,-1,-1,0,1).
%F A266755 a(n) = a(n-2) + a(n-3) + a(n-4) - a(n-5) - a(n-6) - a(n-7) + a(n-9) for n>8. - _Vincenzo Librandi_, Jan 11 2016
%F A266755 a(n) = a(-9-n) for all n in Z. a(n) = a(n+3) for all n in 2Z. - _Michael Somos_, Jan 29 2022
%F A266755 E.g.f.: exp(-x)*(81 - 18*x + exp(2*x)*(107 + 60*x + 6*x^2) + 64*exp(x/2)*cos(sqrt(3)*x/2) + 36*exp(x)*(cos(x) - sin(x)))/288. - _Stefano Spezia_, Mar 05 2023
%F A266755 For n >= 3, if n is even, a(n) = a(n-3) + floor(n/4) + 1, otherwise a(n) = a(n-3). - _Robert FERREOL_, Feb 05 2024
%F A266755 a(n) = floor((n^2+9*n+(3*n+9)*(-1)^n+39)/48). - _Hoang Xuan Thanh_, Jun 03 2025
%e A266755 G.f. = 1 + x^2 + x^3 + 2*x^4 + x^5 + 3*x^6 + 2*x^7 + 4*x^8 + ... - _Michael Somos_, Jan 29 2022
%t A266755 CoefficientList[Series[1/((1-x^2)(1-x^3)(1-x^4)), {x, 0, 100}], x] (* _JungHwan Min_, Jan 10 2016 *)
%t A266755 LinearRecurrence[{0,1,1,1,-1,-1,-1,0,1}, {1,0,1,1,2,1,3,2,4}, 100] (* _Vincenzo Librandi_, Jan 11 2016 *)
%t A266755 Table[Length[Select[IntegerPartitions[n],Length[#]<=n/2&&Max@@#<=3&]],{n,0,30}] (* _Gus Wiseman_, May 23 2021 *)
%t A266755 a[ n_] := Round[(n + 3*(2 - Mod[n,2]))^2/48]; (* _Michael Somos_, Jan 29 2022 *)
%o A266755 (PARI) Vec(1/((1-x^2)*(1-x^3)*(1-x^4)) + O(x^100)) \\ _Michel Marcus_, Jan 11 2016
%o A266755 (PARI) {a(n) = round((n + 3*(2-n%2))^2/48)}; /* _Michael Somos_, Jan 29 2022 */
%o A266755 (Magma) I:=[1,0,1,1,2,1,3,2,4]; [n le 9 select I[n] else Self(n-2)+ Self(n-3)+Self(n-4)-Self(n-5)-Self(n-6)-Self(n-7)+Self(n-9): n in [1..100]]; // _Vincenzo Librandi_, Jan 11 2016
%o A266755 (Sage) (1/((1-x^2)*(1-x^3)*(1-x^4))).series(x, 100).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 13 2019
%Y A266755 Molien series for finite Coxeter groups A_1 through A_12 are A059841, A103221, A266755, A008667, A037145, A001996, and A266776-A266781.
%Y A266755 Molien series for finite Coxeter groups D_3 through D_12 are A266755, A266769, A266768, A003402, and A266770-A266775.
%Y A266755 A variant of A005044.
%Y A266755 Cf. A001400 (partial sums).
%Y A266755 Cf. A308065.
%Y A266755 Number of partitions of n whose Heinz number is in A344293.
%Y A266755 A001399 counts partitions with all parts <= 3, ranked by A051037.
%Y A266755 A025065 counts partitions of n with >= n/2 parts, ranked by A344296.
%Y A266755 A035363 counts partitions of n with n/2 parts, ranked by A340387.
%Y A266755 A110618 counts partitions of n into at most n/2 parts, ranked by A344291.
%Y A266755 Cf. A000041, A008642, A279622, A325691, A344294, A344297.
%K A266755 nonn,easy
%O A266755 0,5
%A A266755 _N. J. A. Sloane_, Jan 10 2016