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 A000625 Number of n-node steric rooted ternary trees; number of n carbon alkyl radicals C(n)H(2n+1) taking stereoisomers into account. (Formerly M1402 N0546) 33
 1, 1, 1, 2, 5, 11, 28, 74, 199, 551, 1553, 4436, 12832, 37496, 110500, 328092, 980491, 2946889, 8901891, 27012286, 82300275, 251670563, 772160922, 2376294040, 7333282754, 22688455980, 70361242924, 218679264772, 681018679604, 2124842137550, 6641338630714, 20792003301836 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Nodes are unlabeled, each node has out-degree <= 3. Steric, or including stereoisomers, means that the children of a node are taken in a certain cyclic order. If the children are rotated it is still the same tree, but any other permutation yields a different tree. See A000598 for the analogous sequence with stereoisomers not counted. Other descriptions of this sequence: steric planted trees with n nodes; total number of monosubstituted alkanes C(n)H(2n+1)-X with n carbon atoms. Let the entries in the nine columns of Blair and Henze's Table I (JACS 54 (1932), p. 1098) be denoted by Ps(n), Pn(n), Ss(n), Sn(n), Ts(n), Tn(n), As(n), An(n), T(n) respectively (here P = Primary, S = Secondary, T = Tertiary, s = stereoisomers, n = non-stereoisomers and the last column T(n) gives total). Then Ps (and As) = A000620, Pn (and An, Sn) = A000621, Ss = A000622, Ts = A000623, Tn = A000624, T = this sequence. Recurrences generating these sequences are given in the Maple program in A000620. REFERENCES J. K. Percus, Combinatorial Methods, Lecture Notes, 1967-1968, Courant Institute, New York University, 212pp. See pp. 64-65. 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 Vaclav Kotesovec, Table of n, a(n) for n = 0..1930 (first 200 terms from T. D. Noe) C. M. Blair and H. R. Henze, The number of stereoisomeric and non-stereoisomeric mono-substitution products of the paraffins, J. Amer. Chem. Soc., 54 (3) (1932), 1098-1106. C. M. Blair and H. R. Henze, The number of stereoisomeric and non-stereoisomeric mono-substitution products of the paraffins, J. Amer. Chem. Soc., 54 (3) (1932), 1098-1105. (Annotated scanned copy) P. Leroux and B. Miloudi, Généralisations de la formule d'Otter, Ann. Sci. Math. Québec, Vol. 16, No. 1, pp. 53-80, 1992. (Annotated scanned copy) G. Polya, Algebraische Berechnung der Anzahl der Isomeren einiger organischer Verbindungen, Zeit. f. Kristall., 93 (1936), 415-443, Eq. (25). G. Polya, Algebraische Berechnung der Anzahl der Isomeren einiger organischer Verbindungen,  Zeit. f. Kristall., 93 (1936), 415-443, Eq. (25). (Annotated scanned copy) 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. [Annotated scanned copy] See p. 44. R. W. Robinson, F. Harary and A. T. Balaban, Numbers of chiral and achiral alkanes and monosubstituted alkanes, Tetrahedron 32 (3) (1976), 355-361 R. W. Robinson, F. Harary and A. T. Balaban, Numbers of chiral and achiral alkanes and monosubstituted alkanes, Tetrahedron 32 (3) (1976), 355-361. (Annotated scanned copy) FORMULA G.f. A(x) = 1 + x + x^2 + 2*x^3 + 5*x^4 + 11*x^5 + 28*x^6 + ... satisfies A(x) = 1 + x*(A(x)^3 + 2*A(x^3))/3. a(0) = a(1) = 1; a(n+1) = 2*a(n/3)/3 + (Sum_{j=1..n} j*a(j)*(Sum_{i=1..n-j} a(i)*a(n-j-i)))/n for n >= 1, where a(k) = 0 if k not an integer (essentially eq (4) in the Robinson et al. paper). - Emeric Deutsch, May 16 2004 a(n) ~ c * b^n / n^(3/2), where b = 3.287112055584474991259... (see A239803), c = 0.346304267394183622435... (see A239810). - Vaclav Kotesovec, Mar 27 2014 MAPLE A := 1; f := proc(n) global A; coeff(series( 1+(1/3)*x*(A^3+2*subs(x=x^3, A)), x, n+1), x, n); end; for n from 1 to 50 do A := series(A+f(n)*x^n, x, n +1); od: A; A000625 := proc(n)     local j, i, a ;     option remember;     if n <= 1 then         1 ;     else         a :=0 ;         for j from 1 to n-1 do             a := a+ j*procname(j)*add(procname(i)*procname(n-j-i-1), i=0..n-j-1) ;         end do:         if modp(n-1, 3) = 0 then             a := a+2*(n-1)*procname((n-1)/3)/3 ;         end if;         a/ (n-1) ;     end if; end proc: seq(A000625(n), n=0..30) ; MATHEMATICA m = 31; c = 1; gf[x_] = Sum[c[k] x^k, {k, 0, m}]; cc = Array[c, m]; coes = CoefficientList[ Series[gf[x] - 1 - (x*(gf[x]^3 + 2*gf[x^3])/3), {x, 0, m}], x] // Rest; Prepend[cc /. Solve[ Thread[ coes == 0], cc][], 1] (* Jean-François Alcover, Jun 24 2011 *) a = a = 1; a[n_Integer] := a[n] = (Sum[j*a[j]*Sum[a[i]*a[n-i-j-1], {i, 0, n-j-1}], {j, 1, n-1}] + (2/3)*(n-1)*a[(n-1)/3])/(n-1); a[_] = 0; Table[a[n], {n, 0, 31}] (* Jean-François Alcover, Apr 21 2016, after Emeric Deutsch *) terms = 32; A[_] = 0; Do[A[x_] = Normal[1 + x*(A[x]^3 + 2*A[x^3])/3 + O[x]^terms], terms]; CoefficientList[A[x], x] (* Jean-François Alcover, Apr 22 2016, updated Jan 11 2018 *) PROG (PARI) a(n) = if(n, my(v=vector(n+1)); v=1; v=1; for(k=1, n-1, v[k+2] = sum(j=1, k, j*v[j+1]*(sum(i=0, k-j, v[i+1]*v[k-j-i+1])))/k + (2/3)*if(k%3, 0, v[k/3+1])); v[n+1], 1) \\ Jianing Song, Feb 17 2019 CROSSREFS Cf. A000598, A000602, A000620-A000624, A000628, A010732, A010733, A086194, A086200, A239803, A239810. Sequence in context: A174145 A124016 A121398 * A202476 A210517 A244531 Adjacent sequences:  A000622 A000623 A000624 * A000626 A000627 A000628 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS Additional comments from Bruce Corrigan, Nov 04 2002 STATUS approved

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Last modified September 24 16:29 EDT 2022. Contains 356943 sequences. (Running on oeis4.)