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 A000084 Number of series-parallel networks with n unlabeled edges. Also called yoke-chains by Cayley and MacMahon. (Formerly M1207 N0466) 42
 1, 2, 4, 10, 24, 66, 180, 522, 1532, 4624, 14136, 43930, 137908, 437502, 1399068, 4507352, 14611576, 47633486, 156047204, 513477502, 1696305728, 5623993944, 18706733128, 62408176762, 208769240140, 700129713630, 2353386723912 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS This is a series-parallel network: o-o; all other series-parallel networks are obtained by connecting two series-parallel networks in series or in parallel. Also the number of unlabeled cographs on n nodes. - N. J. A. Sloane and Eric W. Weisstein, Oct 21 2003 Also the number of P_4-free graphs on n nodes. - Gordon F. Royle, Jul 04 2008 Equals row sums of triangle A144962 and the INVERT transform of A001572. - Gary W. Adamson, Sep 27 2008 See Cameron (1987) p. 165 for a bijection between series-parallel networks and cographs. - Michael Somos, Apr 19 2014 REFERENCES D. E. Knuth, The Art of Computer Programming, 3rd ed. 1997, Vol. 1, p. 589, Answers to Exercises Section 2.3.4.4 5. J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 142. J. Riordan and C. E. Shannon, The number of two-terminal series-parallel networks, J. Math. Phys., 21 (1942), 83-93. Reprinted in Claude Elwood Shannon: Collected Papers, edited by N. J. A. Sloane and A. D. Wyner, IEEE Press, NY, 1993, pp. 560-570. 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). R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.40, notes on p. 133. LINKS N. J. A. Sloane, Table of n, a(n) for n=1..1001 Moussa Abdenbi, Alexandre Blondin Massé, Alain Goupil, On the maximal number of leaves in induced subtrees of series-parallel graphs, Semantic Sensor Networks Workshop 2018, CEUR Workshop Proceedings (2018) Vol. 2113. Antoni Amengual, The intriguing properties of the equivalent resistances of n equal resistors combined in series and in parallel, American Journal of Physics, 68(2), 175-179 (February 2000). - Sameen Ahmed Khan, Mar 06 2010 A. Brandstaedt, V. B. Le and J. P. Spinrad, Graph Classes: A Survey, SIAM Publications, 1999. (For definition of cograph) Peter J. Cameron, Some treelike objects Quart. J. Math. Oxford Ser. (2) 38 (1987), no. 150, 155--183. MR0891613 (89a:05009).  See p. 155. - N. J. A. Sloane, Apr 18 2014 P. J. Cameron, Some sequences of integers, Discrete Math., 75 (1989), 89-102; also in "Graph Theory and Combinatorics 1988", ed. B. Bollobas, Annals of Discrete Math., 43 (1989), 89-102. P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5. S. R. Finch, Series-parallel networks, July 7, 2003. [Cached copy, with permission of the author] Steven R. Finch, Mathematical Constants II, Encyclopedia of Mathematics and Its Applications, Cambridge University Press, Cambridge, 2018. O. Golinelli, Asymptotic behavior of two-terminal series-parallel networks, arXiv:cond-mat/9707023 [cond-mat.stat-mech], 1997. S. Hougardy, Home Page S. Hougardy, Classes of perfect graphs, Discr. Math. 306 (2006), 2529-2571. ISCGI, Cograph graphs Yukinao Isokawa, Series-Parallel Circuits and Continued Fractions, Applied Mathematical Sciences, Vol. 10, 2016, no. 27, 1321 - 1331. Yukinao Isokawa, Listing up Combinations of Resistances, Bulletin of the Kagoshima University Faculty of Education. Bulletin of the Faculty of Education, Kagoshima University. Natural science, Vol. 67 (2016), pp. 1-8. Sameen Ahmed Khan, Mathematica notebook for A048211 and A000084 Sameen Ahmed Khan, The bounds of the set of equivalent resistances of n equal resistors combined in series and in parallel, arXiv:1004.3346 [physics.gen-ph], 2010. S. A. Khan, How Many Equivalent Resistances?, RESONANCE, May 2012. S. A. Khan, Farey sequences and resistor networks, Proc. Indian Acad. Sci. (Math. Sci.) Vol. 122, No. 2, May 2012, pp. 153-162. - From N. J. A. Sloane, Oct 23 2012 Sameen Ahmed Khan, Beginning to count the number of equivalent resistances, Indian Journal of Science and Technology, 2016, Vol 9(44). Z. A. Lomnicki, Two-terminal series-parallel networks, Adv. Appl. Prob., 4 (1972), 109-150. P. A. MacMahon, Yoke-trains and multipartite compositions in connexion with the analytical forms called "trees", Proc. London Math. Soc. 22 (1891), 330-346; reprinted in Coll. Papers I, pp. 600-616. Page 333 gives A000084 = 2*A000669. P. A. MacMahon, The combination of resistances, The Electrician, 28 (1892), 601-602; reprinted in Coll. Papers I, pp. 617-619. Reprinted in Discrete Appl. Math., 54 (1994), 225-228. J. W. Moon, Some enumerative results on series-parallel networks, Annals Discrete Math., 33 (1987), 199-226. J. Riordan and C. E. Shannon, The number of two-terminal series-parallel networks (annotated scanned copy) N. J. A. Sloane, Illustrations of a(1)-a(4) N. J. A. Sloane, First 1001 terms of A000669 and A000084 Marx Stampfli, Bridged graphs, circuits and Fibonacci numbers. Applied Mathematics and Computation. Volume 302, 1 June 2017, Pages 68-79. Takeaki Uno, Ryuhei Uehara and Shin-ichi Nakano, Bounding the Number of Reduced Trees, Cographs, and Series-Parallel Graphs by Compression, in WALCOM: Algorithms and Computation, Lecture Notes in Computer Science, 2012, Volume 7157/2012, 5-16, DOI: 10.1007/978-3-642-28076-4_4. - N. J. A. Sloane, Jul 07 2012 Eric Weisstein's World of Mathematics, Cograph Eric Weisstein's World of Mathematics, Series-Parallel Network FORMULA The sequence satisfies Product_{k>=1} 1/(1-x^k)^A000669(k) = 1 + Sum_{k>=1} a(k)*x^k. a(n) = 2*A000669(n) if n>0. - Michael Somos, Apr 17 2014 a(n) ~ C d^n/n^(3/2) where C = 0.412762889201578063700271574144..., d = 3.56083930953894332952612917270966777... is a root of Product_{n>=1} (1-1/x^n)^(-a(n)) = 2. - Riordan, Shannon, Moon, Rains, Sloane Consider the free algebraic system with two commutative associative operators (x+y) and (x*y) and one generator A. The number of elements with n occurrences of the generator is a(n). - Michael Somos, Oct 11 2006 Examples: n=1: A. n=2: A+A, A*A. n=3: A+A+A, A+(A*A), A*(A+A), A*A*A. EXAMPLE G.f. = x + 2*x^2 + 4*x^3 + 10*x^4 + 24*x^5 + 66*x^6 + 180*x^7 + 522*x^8 + ... The series-parallel networks with 1, 2 and 3 edges are: 1 edge: o-o 2 edges: o-o-o o=o ....................... /\ 3 edges: o-o-o-o o-o=o o--o o-o-o ....................... \/ ..\_/ MAPLE # (continue from A000669): A000084 := n-> if n=1 then 1 else 2*A000669(n); fi; # N denotes all series-parallel networks, S = series networks, P = parallel networks; spec84 := [ N, {N=Union(Z, S, P), S=Set(Union(Z, P), card>=2), P=Set(Union(Z, S), card>=2)} ]: A000084 := n->combstruct[count](spec84, size=n); MATHEMATICA n = 27; s = 1/(1-x) + O[x]^(n+1); Do[s = s/(1-x^k)^Coefficient[s, x^k] + O[x]^(n+1), {k, 2, n}]; CoefficientList[s, x] // Rest (* Jean-François Alcover, Jun 20 2011, updated Jun 30 2015 *) (* faster method: *) sequenceA000084[n_] := Module[{product, x}, product[1] = Series[1/(1 - x), {x, 0, n}]; product[k_] := product[k] = Series[product[k - 1]/(1 - x^k)^Coefficient[ product[k - 1], x^k], {x, 0, n}]; Quiet[Rest[CoefficientList[product[n], x]]]]; sequenceA000084[27] (* Faris Nasybulin, Apr 29 2015 *) PROG (PARI) {a(n) = my(A); if( n<1, 0, A = 1 / (1 - x + x * O(x^n)); for(k=2, n, A /= (1 - x^k + x * O(x^n))^polcoeff(A, k)); polcoeff(A, n))}; /* Michael Somos, Oct 11 2006 */ CROSSREFS Cf. A058351, A058352, A058353, A000311, A006351 (labeled version). See also A058964, A058965. Cf. A144962, A001572. - Gary W. Adamson, Sep 27 2008 Cf. A176500, A176502. - Sameen Ahmed Khan, Apr 27 2010 Sequence in context: A001997 A239605 A309508 * A057734 A151516 A003104 Adjacent sequences:  A000081 A000082 A000083 * A000085 A000086 A000087 KEYWORD nonn,nice,easy AUTHOR EXTENSIONS More decimal places in the third formula added by Vaclav Kotesovec, Jun 24 2014 STATUS approved

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