

A048211


Number of distinct resistances that can be produced from a circuit of n equal resistors using only series and parallel combinations.


18



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
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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 1ohm 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 1ohm resistors, then a resistance 1/x can also be constructed from n 1ohm 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


LINKS

Table of n, a(n) for n=1..25.
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), 175179 (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 (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. 153162.  From N. J. A. Sloane, Oct 23 2012


EXAMPLE

a(2) = 2 since given two 1ohm 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(ni)), 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[ni]}], {f, r[i]}], {i, 1, n/2}]}]; a[n_] := Length[r[n]]; Table[a[n], {n, 1, 15}] (* JeanFrançois 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[nk],
my( r2 = L[nk][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 1ohm 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]
Sequence in context: A055094 A055729 A238826 * A098719 A274289 A265023
Adjacent sequences: A048208 A048209 A048210 * A048212 A048213 A048214


KEYWORD

nonn,nice,more,hard


AUTHOR

Tony Bartoletti


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


STATUS

approved



