# H

An nxn matrix with entries either 1 or -1 where each row is orthogonal to every other row. Of course, a row's inner product with itself is n. So when you multiply a Hadamard matrix times it's transpose you get n times the identity matrix.

## Hamiltonian Chain

A chain in a graph such that the nodes of the chain comprise all the nodes of the graph.

## Hamiltonian Cycle

A cycle in a graph such that the nodes of the cycle comprise all the nodes of the graph. ## Hamming Distance

In a binary code a distance can be defined between code words, given two words v and w the Hamming distance, d(v,w) is the number of positions in which they differ.

For instance:

```          0100110110111   and
0010110110111
```
are at Hamming distance 2 from one another.

## Homeomorph

A graph obtained from another by inserting new nodes along existing edges. The following graph is a homeomorph of K(5) . ## Hypercube

A regular polytope, the convex hull of the points (e1,...,ed) where each coordinate ei is either 0 or 1.

## Hyperplane

A (d-1)-dimensional flat in Rd.

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

An On-line Dictionary of Combinatorics