## Friday, June 29, 2012

### Linear ordinary differential operators

Linear ordinary differential operators are endomorphisms of differentiable vector spaces, and they can be decomposed into separate functions that handle the complementary solution $y_c$ and the particular solution $y_p$ of a differential vector space.

Linear differential operators are inverted by linear integral operators, for example, the diff operator is inverted by antidiff. A variety of methods can be used to make linear differential operators invertible including laplace transforms and series methods.

## Monday, June 25, 2012

### Commutative decomposition of endofunctions

All endofunctions can be described as the composition of zap operations. If two different places are disjoint, then zap operations involving them commute. For example, the cube function only effects the magnitude of a number and - only effects its sign so these functions commute:
(= (compose (polynomial. [0 0 0 1]) (polynomial. [0 -1]))
(compose (polynomial. [0 -1]) (polynomial. [0 0 0 1])))

Doubling a number can be described as incrementing the multiplicity of the prime number factor 2 of the number, so multiplication commutes and $\mathbb{Q}+$ forms an abelian group. In general, two polynomials commute if they are either powers of x or chebyshev polynomials. First order homogeneous linear ordinary differential operators also commute under composition:
(= (compose (lodo. [0 1 1]) (lodo. [0 2 1]))
(compose (lodo. [0 2 1]) (lodo. [0 1 1])))

The behavior of the composition of linear ordinary differential operators corresponds to polynomial multiplication. These operators can be described as endomorphisms of vector spaces of differentiable functions.

## Thursday, June 14, 2012

### Arithmetic module

The arithmetic module deals with functions that are built out of the of addition, multiplication, and composition. All of these higher order operations have derivative laws associated with them: the sum rule, the product rule, and the chain rule. As such, this module includes derivative operations.

Entire functions are described entirely using power series. The functions exp, sin, cos, sinh, cosh, erfi, the reciprocal of gamma, all polynomials, and all addition, multiplication, and compositions of entire functions are entire. However, entire functions are not closed under inversion or division so ln, sqrt, and tan are not entire.

Linear ordinary differential operators receive functions as arguments and then they output new functions using higher order operations and differentiation. Linear ordinary differential operators are closed under composition, so they form their compositional monoid. The derivative function itself is a differential operator that uses a hidden multimethod internally that dispatches on the type of its arguments.

## Thursday, June 7, 2012

### The regularity of relations

The regularity of relations first leads us to functions which are relations with a out degree regularity of one. Functions that also have an in degree regularity are place forms, and if the in degree regularity of a function is one then the function is one-to-one. Since bijections are just special cases of place forms you can apply all place form functions to them:
(setf [reverse coll] '(3 2 1))
;=> (1 2 3)

Functions that cannot be expressed as place forms are in degree irregular. Count is an example of such a function, because there is only one type of empty collection, and there are many other possible collections for the other sizes. Monoids are also irregular functions because there is a different number of partitions for different objects in most monoids, unless you consider the identity.