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 A048211 Number of distinct resistances that can be produced from a circuit of n equal resistors using only series and parallel combinations. 19
 1, 2, 4, 9, 22, 53, 131, 337, 869, 2213, 5691, 14517, 37017, 93731, 237465, 601093, 1519815, 3842575, 9720769, 24599577, 62283535, 157807915, 400094029, 1014905643, 2576046289, 6541989261, 16621908599 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Found by exhaustive search. Program produces all values that are combinations of two binary operators a() and b() (here "sum" and "reciprocal sum of reciprocals") over n occurrences of 1. E.g., given 4 occurrences of 1, the code forms all allowable postfix forms, such as 1 1 1 1 a a a and 1 1 b 1 1 a b, etc. Each resulting form is then evaluated according to the definitions for a and b. Each resistance that can be constructed from n 1-ohm resistors in a circuit can be written as the ratio of two positive integers, neither of which exceeds the (n+1)st Fibonacci number. E.g., for n=4, the 9 resistances that can be constructed can be written as 1/4, 2/5, 3/5, 3/4, 1/1, 4/3, 5/3, 5/2, 4/1 using no numerator or denominator larger than Fib(n+1) = Fib(5) = 5. If a resistance x can be constructed from n 1-ohm resistors, then a resistance 1/x can also be constructed from n 1-ohm resistors. - Jon E. Schoenfield, Aug 06 2006 The fractions in the comment above are a superset of the fractions occurring here, corresponding to the upper bound A176500. - Joerg Arndt, Mar 07 2015 The terms of this sequence consider only series and parallel combinations; A174283 considers bridge combinations as well. - Jon E. Schoenfield, Sep 02 2013 REFERENCES Sameen Ahmed Khan, Beginning to count the number of equivalent resistances, Indian Journal of Science and Technology, 2016, Vol 9(44), DOI: 10.17485/ijst/2016/v9i44/88086, http://www.indjst.org/index.php/indjst/article/viewFile/88086/75398 LINKS 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). [From Sameen Ahmed Khan, Apr 27 2010] Sameen Ahmed Khan, Mathematica program 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. - From N. J. A. Sloane, Oct 15 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 Marx Stampfli, Bridged graphs, circuits and Fibonacci numbers, Applied Mathematics and Computation, Volume 302, 1 June 2017, Pages 68-79. EXAMPLE a(2) = 2 since given two 1-ohm resistors, a series circuit yields 2 ohms, while a parallel circuit yields 1/2 ohms. MAPLE r:= proc(n) option remember; `if`(n=1, {1}, {seq(seq(seq(       [f+g, 1/(1/f+1/g)][], g in r(n-i)), f in r(i)), i=1..n/2)})     end: a:= n-> nops(r(n)): seq(a(n), n=1..15);  # Alois P. Heinz, Apr 02 2015 MATHEMATICA r[n_] := r[n] = If[n == 1, {1}, Union @ Flatten @ {Table[ Table[ Table[ {f+g, 1/(1/f+1/g)}, {g, r[n-i]}], {f, r[i]}], {i, 1, n/2}]}]; a[n_] := Length[r[n]]; Table[a[n], {n, 1, 15}] (* Jean-François Alcover, May 28 2015, after Alois P. Heinz *) PROG (PARI) \\ not efficient; just to show the method N=10; L=vector(N);  L[1]=[1]; { for (n=2, N,     my( T = Set( [] ) );     for (k=1, n\2,         for (j=1, #L[k],             my( r1 = L[k][j] );             for (i=1, #L[n-k],                 my( r2 = L[n-k][i] );                 T = setunion(T,  Set([r1+r2, r1*r2/(r1+r2) ]) );             );         );     );     T = vecsort(Vec(T), , 8);     L[n] = T; ); } for(n=1, N, print1(#L[n], ", ") ); \\ Joerg Arndt, Mar 07 2015 CROSSREFS Let T(x, n) = 1 if x can be constructed with n 1-ohm resistors in a circuit, 0 otherwise. Then A048211 is t(n) = sum(T(x, n)) for all x (x is necessarily rational). Let H(x, n) = 1 if T(x, n) = 1 and T(x, k) = 0 for all k < n, 0 otherwise. Then A051389 is h(n) = sum(H(x, n)) for all x (x is necessarily rational). Cf. A153588, A174283, A174284, A174285 and A174286, A176497, A176498, A176499, A176500, A176501, A176502. - Sameen Ahmed Khan, Apr 27 2010 Cf. A180414. Sequence in context: A055729 A317735 A238826 * A098719 A274289 A265023 Adjacent sequences:  A048208 A048209 A048210 * A048212 A048213 A048214 KEYWORD nonn,nice,more,hard AUTHOR EXTENSIONS More terms from John W. Layman, Apr 06 2002 a(16)-a(21) from Jon E. Schoenfield, Aug 06 2006 a(22) from Jon E. Schoenfield, Aug 28 2006 a(23) from Jon E. Schoenfield, Apr 18 2010 Definition edited (to specify that the sequence considers only series and parallel combinations) by Jon E. Schoenfield, Sep 02 2013 a(24)-a(25) from Antoine Mathys, Apr 02 2015 a(26)-a(27) from Johannes P. Reichart, Nov 24 2018 STATUS approved

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Last modified December 9 17:51 EST 2018. Contains 318023 sequences. (Running on oeis4.)