The characteristic that distinguishes finite group theory from other branches of algebra is the nature of many important theorems. Indeed, proofs in finite group theory often amount to counting; Lagrange's theorem and the Sylow theorems are quintessential examples of the genre. Another highly appealing arithmetical result that is typically presented as an application of the Sylow theorems is that a group of order pq, where p and q are primes, q < p and q does not divide p 1, is cyclic (see, for example, [1, p. 245], [2, p. 358] or [3, p. 495]). The standard proof of this statement is as follows. By the Sylow theorems there are subgroups H and K of orders p and q that are normal. From the normality of H and K and the fact that H n K = {e}, it follows that elements of H and K commute with each other (see [2, p. 358]). Thus, if IhI = p and Ik I = q, then I hk I = and the group is cyclic. Since the Sylow theorems occur late in many undergraduate abstract algebra textbooks, most students who take only one semester of algebra will not encounter them. In this note we offer a proof of the pq result that does not use the Sylow theorems or normality. It can be given as an application of cosets and Lagrange's theorem. Our argument is an excellent illustration of sophisticated counting. The proof utilizes the following easy-to-prove facts.
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