Theory of Series in the $\mathcal{A}$-calculus and the $N$-Pythagorean Theorem

Abstract

In this paper we study sequences, series, power series and uniform convergence in the $\mathcal{A}$-Calculus. Here $\mathcal{A}$ denotes an associative unital real algebra. We say a function is $\mathcal{A}$-differentiable if it is real differentiable and its differential is in the regular representation of the algebra. We show the theory of sequences and numerical series resembles the usual theory, but, the proof to establish this claim requires modification of the standard arguments due to the submultiplicativity of the norm on $\mathcal{A}$. In contrast, the theorems concerning divergence of power series over $\mathcal{A}$ are modified notably from the standard theory. We study how the ratio, root and geometric series results are modified due to both the submultiplicativity of the norm and the calculational novelty of zero-divisors. Despite these difficulties, we find natural generalizations of the usual theorems of real analysis for uniformly convergent series of functions. We prove the usual calculations with power series transfer nicely to the $\mathcal{A}$-calculus. Then power series are used to define sine, cosine, hyperbolic sine, hyperbolic cosine and the exponential. Many standard identities are derived for entire functions on an arbitrary commutative $\mathcal{A}$. Finally, special functions are introduced and we derive the $N$-Pythagorean Theorem which produces $\cos^2 \theta+ \sin^2 \theta=1$ and $\cosh^2 \theta - \sinh^2 \theta = 1$ as well as a host of other less known identities.