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A221918 Triangle of denominators of sum of two unit fractions: 1/n + 1/m, n >= m >= 1. 8
1, 2, 1, 3, 6, 3, 4, 4, 12, 2, 5, 10, 15, 20, 5, 6, 3, 2, 12, 30, 3, 7, 14, 21, 28, 35, 42, 7, 8, 8, 24, 8, 40, 24, 56, 4, 9, 18, 9, 36, 45, 18, 63, 72, 9, 10, 5, 30, 20, 10, 15, 70, 40, 90, 5, 11, 22, 33, 44, 55, 66, 77, 88, 99, 110, 11, 12, 12, 12, 3, 60, 4, 84, 24, 36, 60, 132, 6 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The corresponding triangle of numerators is A221919.

The law for the electrical resistance in a parallel circuit with two resistors R1 and R2 is 1/R = 1/R1 + 1/R2. Here we take 1/R(n,m) =  1/n + 1/m, with n >= m> =1, and R(n,m) = a(n,m)/A221919(n,m).

The reduced mass mu in a two body problem with masses m1 and m2 is given by 1/mu = 1/m1 + 1/m2.

The radius R of the twin circles of Archimedes' arbelos with the radii of the two small half-circles r1 and r2 is given by 1/R = 1/r1 +1/r2. The large half-circle has radius r =  r1 + r2. See, e.g., the Bankoff reference (according to which one should speak of a triple of such radius R circles). There are much more such radius R circles. See the Arbelos references given by Schoch, especially reference [3].

The columns give A000027, A145979(n-2), A221920, A221921, A222463 for m = 1, 2, ..., 5.

This and the companion entry resulted from a remark on the twin circles in Archimedes' arbelos in the Strick reference, p. 13, and the obvious question about their radii and centers. See the MathWorld link, also for more references.

The rationals R(n,m) = a(n,m)/A221919(n,m) (in lowest terms) equal H(n,m)/2, where H(n,m) = A227041(n,m)/A227042(n,m) is the harmonic mean of m and n. - Wolfdieter Lang, Jul 02 2013

REFERENCES

L. Bankoff, Are the Twin Circles of Archimedes Really Twins?, Mathematics Mag. 47,4 (1974) 214-218.

H. K. Strick, Geschichten aus der Mathematik, Spektrum der Wissenschaft - Spezial 2/2009.

LINKS

Table of n, a(n) for n=1..78.

T. Schoch, Arbelos References.

Eric W. Weisstein, Arbelos (MathWorld).

FORMULA

a(n,m) = denominator(1/n +1/m) = numerator(n*m/(n+m)), n >= m >= 1 and 0 otherwise.

a(n,m)/A221919(n,m) = R(n,m) = n*m/(n+m). 1/R(n,m) = 1/n + 1/m.

EXAMPLE

The triangle a(n,m) begins:

n\m    1    2    3    4    5    6   7   8   9   10   11  12 ...

1:     1

2:     2    1

3:     3    6    3

4:     4    4   12    2

5:     5   10   15   20    5

6:     6    3    2   12   30    3

7:     7   14   21   28   35   42   7

8:     8    8   24    8   40   24  56   4

9:     9   18    9   36   45   18  63  72   9

10:   10    5   30   20   10   15  70  40  90    5

11:   11   22   33   44   55   66  77  88  99  110   11

12:   12   12   12    3   60    4  84  24  36   60  132   6

...

a(n,1) = n because 1/R(n,1) =  1/n +1/1 = (n+1)/n, hence a(n,1) = denominator(1/n +1/1/) = n =  numerator(R(n,1)).

a(5,3) = denominator(1/5 + 1/3) = denominator(8/15 ) = 15.

a(6,3) = denominator(1/6 + 1/3) = denominator(9/18 ) = denominator(1/2) = 2.

The triangle of rationals R(n,m) = n*m/(n+m) = a(n,m)/A221919(n,m) given by 1/R(n,m) = 1/n + 1/m starts:

n\m    1    2      3     4      5     6     7     8     9   10

1:    1/2

2:    2/3    1

3:    3/4   6/5   3/2

4:    4/5   4/3  12/7    2

5:    5/6  10/7  15/8   20/9   5/2

6:    6/7   3/2    2    12/5  30/11   3

7:    7/8  14/9  21/10  28/11 35/12 42/13  7/2

8:    8/9   8/5  24/11   8/3  40/13 24/7  56/15   4

9:   9/10  18/11  9/4   36/13 45/14 18/5  63/16 72/17  9/2

10:  10/11  5/3  30/13  20/7  10/3  15/4  70/17 40/9  90/19  5

...

MATHEMATICA

a[n_, m_] := Denominator[1/n + 1/m]; Table[a[n, m], {n, 1, 12}, {m, 1, n}] // Flatten  (* Jean-Fran├žois Alcover, Feb 25 2013 *)

CROSSREFS

Cf. A221919 (companion).

Sequence in context: A289815 A125205 A125206 * A193897 A226122 A133904

Adjacent sequences:  A221915 A221916 A221917 * A221919 A221920 A221921

KEYWORD

nonn,easy,tabl,frac

AUTHOR

Wolfdieter Lang, Feb 21 2013

STATUS

approved

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Last modified February 25 19:39 EST 2021. Contains 341618 sequences. (Running on oeis4.)