## 2228 Reputation

17 years, 169 days

## Type checking for a certain number of el...

In analogy with something like

```type(<1,2>,'Vector'(2,posint));
```

it would have been nice if something like

```type({1,2},'set'(2,posint));
type([1,2],'list'(2,posint));
```

was possible. How can one, using `type`, check for a certain number of elements in sets and lists?

## Indexing functions of Arrays with arbitr...

Maple

Using the procedure `permsPosNeg` as given in the blog entry Positive and negative permutations, including the improvement obtained in the blog entry Refactoring Maple code, consider the following procedure:

## Positive and negative permutations...

In connection with tensors, I would like to be able to treat arbitrary symmetries (positive permutations) and antisymmetries (negative permutations) of the entries of an Array.

An example is the Riemannian curvature tensor which has the following positive and negative permutations:

## Cross products and division algebras...

The recent thread A crossprod problem, although not directly related, inspired me to write this blog entry about bilinear cross products and their noticable property [see for instance P. Lounesto, Clifford Algebras and Spinors, 2nd ed. (Cambridge University Press, Cambridge, 2001)]:

Theorem: A bilinear cross product obeying the orthogonality property and the Pythagorean property, see below, exists only in `3` or `7` (real or complex) dimensions.

## Why andmap and ormap ?...

In working out the procedure `isDeeplyAlgebraic` in the post Deeply algebraic, I was trying to use something like ``and`` and ``or``, analogously to ``+``, ``*``, ``.``, etc.

 1 2 3 4 5 6 7 Last Page 2 of 12
﻿