On the History of the Square and Multiply Algorithm

Abstract

The square-and-multiply algorithm, also known as binary exponentiation or repeated squaring, is a standard method for fast exponentiation in modern computation. Its historical origins, however, remain uncertain. This paper examines the emergence and progressive formalization of the method through selected primary sources. Particular attention is given to Jamshid al-Kashi's fifteenth-century Miftah al-Hisab, where the procedure is presented explicitly as a general computational method and claimed by al-Kashi as his own innovation. Earlier instances of successive squaring are identified in the works of al-Uqlidisi and al-Biruni, although in these cases the technique appears in particular calculations rather than as a fully articulated general rule. The earliest known antecedent is found in Pingala's prosodic studies in ancient India (c. 200 BCE), which seem to presuppose the conceptual basis of the method in their use of binary representation. As part of the historical development, Legendre's 1798 worked example is one of the earliest documented European use of the algorithm which appears as a subordinate step within a specific number-theoretic computation. The evidence suggests not a single continuous line of transmission, but the repeated independent reappearance of related procedures in distinct contexts. By the twentieth century, square-and-multiply became a special case within the broader theory of addition chains. By exploring this intellectual progression, this paper sheds some light on the historical background of an algorithm that is prominent in modern computation.