# Leibniz harmonic triangle

The Leibniz harmonic triangle is a triangular arrangement of fractions in which the outermost diagonals consist of the reciprocals of the row numbers and each inner cell is the absolute value of the cell above minus the cell to the left. To put it algebraically, $L(r,1)=\frac{1}{n}$ (where $r$ is the number of the row, starting from 1, and $c$ is the column number, never more than $r$) and $L(r,c)=L(r-1,c-1)-L(r,c-1)$.

The first eight rows are:

 $\begin{array}[]{cccccccccccccccccc}&&&&&&&&&1&&&&&&&&\\ &&&&&&&&\frac{1}{2}&&\frac{1}{2}&&&&&&&\\ &&&&&&&\frac{1}{3}&&\frac{1}{6}&&\frac{1}{3}&&&&&&\\ &&&&&&\frac{1}{4}&&\frac{1}{12}&&\frac{1}{12}&&\frac{1}{4}&&&&&\\ &&&&&\frac{1}{5}&&\frac{1}{20}&&\frac{1}{30}&&\frac{1}{20}&&\frac{1}{5}&&&&\\ &&&&\frac{1}{6}&&\frac{1}{30}&&\frac{1}{60}&&\frac{1}{60}&&\frac{1}{30}&&\frac% {1}{6}&&&\\ &&&\frac{1}{7}&&\frac{1}{42}&&\frac{1}{105}&&\frac{1}{140}&&\frac{1}{105}&&% \frac{1}{42}&&\frac{1}{7}&&\\ &&\frac{1}{8}&&\frac{1}{56}&&\frac{1}{168}&&\frac{1}{280}&&\frac{1}{280}&&% \frac{1}{168}&&\frac{1}{56}&&\frac{1}{8}&\\ &&&&&\vdots&&&&\vdots&&&&\vdots&&&&\\ \end{array}$

The denominators are listed in A003506 of Sloane’s OEIS, while the numerators, which are all 1s, are listed in A000012. The denominators of the second outermost diagonal are oblong numbers. The sum of the denominators in the $n$th row is $n2^{n-1}$.

Just as Pascal’s triangle can be computed by using binomial coefficients, so can Leibniz’s:

 $L(r,c)=\frac{1}{c{r\choose c}}$

.

This triangle can be used to obtain examples for the Erdős-Straus conjecture (http://planetmath.org/ErdHosStrausConjecture) when $4|n$.

## References

• 1 A. Ayoub, “The Harmonic Triangle and the Beta FunctionMath. Magazine 60 4 (1987): 223 - 225
• 2 D. Darling, “Leibniz’ harmonic triangle” in The Universal Book of Mathematics: From Abracadabra To Zeno’s paradoxes. Hoboken, New Jersey: Wiley (2004)
Title Leibniz harmonic triangle LeibnizHarmonicTriangle 2013-03-22 16:47:21 2013-03-22 16:47:21 PrimeFan (13766) PrimeFan (13766) 5 PrimeFan (13766) Definition msc 05A10 Leibniz’ harmonic triangle Leibniz’s harmonic triangle Leibniz’z harmonic triangle