between 104 and 107. Despite over 2,000 years of study, it still is not known whether there exist infinitely many perfect numbers; or whether there are any odd ones. Today the term "perfect number" is merely historic in nature, used for the sake of tradition. These peculiar numbers had received the name on account of their analogy to the construction of man, who was held to be nature's most perfect creation, and above all on account of their own peculiar regularity. Thus, they had been so named on the same grounds as perfect objects in nature, and perfectly proportioned edifices and statues created by man; the numbers had come to be called "perfect" in order to emphasize their special regularity. The Greek mathematicians had named these numbers "perfect" in the same sense in which philosophers and artists used the word. Jamblich (In Nicomachi arithmeticam, Leipzig, 1894) states that the Pythagoreans had called the number 6 "marriage", "health", and "beauty", on account of the harmony and accord of that number. The perfect numbers early on came to be treated as the measure of other numbers: those in which the sum of the divisors is greater than the number itself, as in 12, have — since as early as Theon of Smyrna, ca. 130 A.D. — been called "redundant" (), "more than perfect" (plus quam perfecti), or "abundant numbers", and those the sum of whose divisors is smaller, as in 8, have been called "deficient numbers" (deficientes). As of 7 December 2018, 51 perfect numbers had been identified. Physics and chemistry A variety of physical and chemical concepts include, in their names, the word "perfect." Physicists designate as a perfectly rigid body, one that "is not deformed by forces applied to it", in full awareness that this is a fictitious body, that no such body exists in nature. The concept is an ideal construct. A perfectly plastic body is one that is deformed infinitely at a constant load corresponding to the body's limit of plasticity: this is a physical model, not a body observed in nature. A perfectly black body would be one that absorbed completely, radiation falling upon it — that is, a body with a coefficient of absorption equal to unity. A crystal is perfect when its physically equivalent walls are equally developed; it has a perfect structure when it answers the requirements of spatial symmetry and is free of