0,5

Trees are unrooted, nodes are unlabeled. Every node has degree <= 4.

Ignoring stereoisomers means that the children of a node are unordered. They can be permuted in any way and it is still the same tree. See A000628 for the analogous sequence with stereoisomers counted.

In alkanes every carbon has valence exactly 4 and every hydrogen has valence exactly 1. But the trees considered here are just the carbon "skeletons" (with all the hydrogens stripped off) so now each carbon bonds to 1 to 4 other carbons. The degree of each node is then <= 4.

M. van Almsick, H. Dolhaine and H. Honig, Efficient algorithms to enumerate isomers and diamutamers with more than one type of substituent, J. Chem. Info. and Computer Science, 40 (2000), 956-966.

A. T. Balaban, J. W. Kennedy and L. V. Quintas, The number of alkanes having n carbons and a longest chain of length d, J. Chem. Education, 65 (No. 4, 1988), 304-313.

F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Camb. 1998, p. 290.

L. Bytautas and D. J. Klein, Chemical combinatorics for alkane-isomer enumeration and more, J. Chem. Inf. Comput. Sci., 38 (1998), 1063-1078.

A. Cayley, Ueber die analytischen Figuren, welche in der Mathematik Baeume genannt werden..., Chem. Ber. 8 (1875), 1056-1059.

R. Davies and P. J. Freyd, C_{167}H_{336} is The Smallest Alkane with More Realizable Isomers than the Observable Universe has Particles, Journal of Chemical Education, Vol. 66, 1989, pp. 278-281.

J. L. Faulon, D. Visco and D. Roe, Enumerating Molecules, In: Reviews in Computational Chemistry Vol. 21, Ed. K. Lipkowitz, Wiley-VCH, 2005.

J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 529.

Handbook of Combinatorics, North-Holland '95, p. 1963.

F. Harary and R. Z. Norman, Dissimilarity characteristic theorems for graphs, Proc. Amer. Math. Soc. 11 (1960), 332-334.

J. B. Hendrickson and C. A. Parks, "Generation and Enumeration of Carbon skeletons", J. Chem. Inf. Comput. Sci, vol. 31 (1991) pp. 101-107. See Table 2, column 2 on page 103.

H. R. Henze and C. M. Blair, The number of isomeric hydrocarbons of the methane series, J. Amer. Chem. Soc., 53 (1931), 3077-3085.

H. R. Henze and C. M. Blair, The number of structurally isomeric hydrocarbons of the ethylene series, J. Amer. Chem. Soc., 55 (1933), 680-686.

M. D. Jackson and T. I. Bieber, Applications of degree distribution, 2: construction and enumeration of isomers in the alkane series, J. Chem. Info. and Computer Science, 33 (1993), 701-708.

E. V. Konstantinova and M. V. Vidyuk, Discriminating tests of information and toplogical indices; animals and trees; J. Chem. Inf. Comput. Sci., 43 (2003), 1860-1871.

J. Lederberg et al., Applications of artificial intelligence for chemical systems, I: The number of possible organic compounds. Acyclic structures containing C, H, O and N, J. Amer. Chem. Soc., 91 (1969), 2973-2097.

P. Leroux and B. Miloudi, ``G\'{e}n\'{e}ralisations de la formule d'Otter,'' Ann. Sci. Math. Qu\'{e}bec, Vol. 16, No. 1, pp. 53-80, 1992.

S. M. Losanitsch, Die Isomerie-Arten bei den Homologen der Paraffin-Reihe, Chem. Ber. 30 (1897), 1917-1926.

L. M. Masinter, Applications of artificial intelligence for chemical systems, XX, Exhaustive generation of cyclic and acyclic isomers, J. Amer. Chem. Soc., 96 (1974), 7702-7714.

W. R. Mueller et al., Molecular topological index, J. Chem. Inf. Comput. Sci., 30 (1990),160-163.

D. Perry, The number of structural isomers ..., J. Amer. Chem. Soc. 54 (1932), 2918-2920. [Gives a(60) correctly - compare first link below]

M. Petkovsek and T. Pisanski, Counting disconnected structures: chemical trees, fullerenes, I-graphs and others, Croatica Chem. Acta, 78 (2005), 563-567.

G. Polya, Algebraische Berechnung der Anzahl der Isomeren einiger organischer Verbindungen, Zeit. f. Kristall., 93 (1936), 415-443.

R. C. Read, The Enumeration of Acyclic Chemical Compounds, pp. 25-61 of A. T. Balaban, ed., Chemical Applications of Graph Theory, Ac. Press, 1976; see p. 28.

R. W. Robinson, F. Harary and A. T. Balaban, Numbers of chiral and achiral alkanes..., Tetrahedron 32 (1976), 355-361.

ten Hoor, Education in Chemistry (details needed).

N. Trinajstich, Z. Jerievi, J. V. Knop, W. R. Muller and K. Szymanski, COMPUTER GENERATION OF ISOMERIC STRUCTURES, Pure & Appl. Chem., Vol. 55, No. 2, pp. 379-39O, 1983.

S. Wagner, Graph-theoretical enumeration and digital expansions: an analytic approach, Dissertation, Fakult. f. Tech. Math. u. Tech. Physik, Tech. Univ. Graz, Austria, Feb., 2006.

N. J. A. Sloane, Table of n, a(n) for n = 0..60

R. Aringhieri, P. Hansen and F. Malucelli, Chemical Tree Enumeration Algorithms, Report TR-99-09, Dept. Informatica, Univ. Pisa, 1999.

H. Bottomley, Illustration of initial terms of A000022, A000200, A000602

Michael A. Kappler, GENSMI: Exhaustive Enumeration of Simple Graphs. Daylight CIS, Inc., EuroMUG '04;4-5 Nov, 2004.

E. M. Rains and N. J. A. Sloane, On Cayley's Enumeration of Alkanes (or 4-Valent Trees)., J. Integer Sequences, Vol. 2 (1999), Article 99.1.1.

N. J. A. Sloane, Maple program and first 60 terms for A000022, A000200, A000598, A000602, A000678

Index entries for sequences related to trees

Index entries for "core" sequences

a(n) = A010372(n) + A010373(n/2) for n even, a(n) = A010372(n) for n odd.

Also equals A000022 + A000200 (n>0), both of which have known generating functions. Also g.f. = A000678(x)-A000599(x)+A000598(x^2) = (x+x^2+2x^3...)-(x^2+x^3+3x^4...)+(1+x^2+x^4+...) = 1+x+x^2+x^3+2x^4+3x^5...

a(6)=5 because hexane has five isomers: n-hexane, 2-methylhexane, 3-methylhexane, 2,2-dimethylhexane, 2,3-dimethylhexane. - Michael Lugo (mtlugo(AT)mit.edu), Mar 15 2003

A000602 := proc(n) if n=0 then RETURN(1) else A000022(n)+A000200(n); fi; end;

Cf. A000598, A000625, A000628, A067608-A067610.

Sequence in context: A047031 A056766 A080091 this_sequence A034790 A047121 A096753

Adjacent sequences: A000599 A000600 A000601 this_sequence A000603 A000604 A000605

nonn,easy,core,nice

njas

Additional comments from Steve Strand (snstrand(AT)comcast.net), Aug 20, 2003.

Kappler reference from Jonathan Vos Post (jvospost2(AT)yahoo.com), Dec 15 2005