OFFSET

1,2

COMMENTS

If x and y require xn and yn resistors respectively, then (x+y) and 1/(1/x + 1/y) require no more than (xn+yn). Inspired by a sci.math posting by Miguel A. Lerma (lerma(AT)math.nwu.edu).

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 a(n) is h(n) = sum(H(x, n)) for all x (x is necessarily rational).

LINKS

Miguel A. Lerma, resistors, post in the newsgroup sci.math, Nov 5 1999.

FORMULA

EXAMPLE

a(5)=card({5,1/5,5/4,4/5,7/3,3/7,7/4,4/7,7/2,2/7,7/5,5/7,8/3,3/8,8/5,5/8, 5/6,7/6,6/5,7/6}). E.g. 6/5 is made from two resistors in series in parallel with three resistors in series, since 6/5 = 1/(1/2 + 1/3).

CROSSREFS

KEYWORD

nonn,nice,more,changed

AUTHOR

EXTENSIONS

a(15)-a(21) from Jon E. Schoenfield, Aug 28 2006

Definition corrected by Jon E. Schoenfield, Aug 27 2006

a(22)-a(23) from Graeme McRae, Aug 18 2007

a(24)-a(25) from Antoine Mathys, Mar 20 2017

Definition changed to say "exactly". - N. J. A. Sloane, Nov 07 2020

a(26) from Antoine Mathys, Nov 05 2024

STATUS

approved