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An egg-shaped curve. Lockwood (1967) calls the negative pedal curve of an ellipse with eccentricity e<=1/2 an ovoid.

A symbol employed in a formal propositional calculus.

A popular acronym for "principal ideal domain." In engineering circles, the acronym PID refers to the "proportional-integral-derivative method" algorithm for controlling ...

The fraction eta of a volume filled by a given collection of solids.

Given an amicable pair (m,n), the quantity sigma(m) = sigma(n) (1) = =s(m)+s(n) (2) = m+n (3) is called the pair sum, where sigma(n) is the divisor function and s(n) is the ...

The term pairwise refers to taking all unordered 2-subsets of a given set. For example, the pairwise products of the set {a,b,c} are ab, ac, and bc.

The Paley class of a positive integer m=0 (mod 4) is defined as the set of all possible quadruples (k,e,q,n) where m=2^e(q^n+1), (1) q is an odd prime, and k={0 if q=0; 1 if ...

Proved in 1933. If q is an odd prime or q=0 and n is any positive integer, then there is a Hadamard matrix of order m=2^e(q^n+1), where e is any positive integer such that ...

The two-dimensional version of the ham sandwich theorem.

A papal cross is a cross having three crossbars. A schematic polyomino version of a papal cross is illustrated above.