A pronic number is a number which is the product of two consecutive integers, that is, a number of the form. The study of these numbers dates back to Aristotle. They are also called oblong numbers, heteromecic numbers, or rectangular numbers; however, the term "rectangular number" has also been applied to the composite numbers. The first few pronic numbers are: If n is a pronic number, then the following is true:
The pronic numbers were studied as figurate numbers alongside the triangular numbers and square numbers in Aristotle's Metaphysics, and their discovery has been attributed much earlier to the Pythagoreans. As a kind of figurate number, the pronic numbers are sometimes called oblong because they are analogous to polygonal numbers in this way: The th pronic number is twice the th triangular number and more than the th square number, as given by the alternative formula for pronic numbers. The th pronic number is also the difference between the odd square and the st centered hexagonal number.
Sum of pronic numbers
The sum of the reciprocals of the pronic numbers is a telescoping series that sums to 1: The partial sum of the first terms in this series is The partialsum of the first pronic numbers is twice the value of the th tetrahedral number:
Additional properties
The th pronic number is the sum of the first even integers. All pronic numbers are even, and 2 is the only prime pronic number. It is also the only pronic number in the Fibonacci sequence and the only pronic Lucas number. The number of off-diagonal entries in a square matrix is always a pronic number. The fact that consecutive integers are coprime and that a pronic number is the product of two consecutive integers leads to a number of properties. Each distinct prime factor of a pronic number is present in only one of the factors n or n+1. Thus a pronic number is squarefreeif and only if and are also squarefree. The number of distinct prime factors of a pronic number is the sum of the number of distinct prime factors of and. If 25 is concatenated to the decimal representation of any pronic number, the result is a square number e.g. 625 = 252, 1225 = 352. This is because
