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A heptomino in the shape of the Greek character pi.

A braid in which strands are intertwined in the center and are free in "handles" on either side of the diagram.

For s_1,s_2=+/-1, lim_(epsilon_1->0; epsilon_2->0)1/(x_1-is_1epsilon_1)1/(x_2-is_2epsilon_2) =[PV(1/(x_1))+ipis_1delta(x_1)][PV(1/(x_2))+ipis_2delta(x_2)] ...

The Betti numbers of a compact orientable n-manifold satisfy the relation b_i=b_(n-i).

The metric ds^2=(dx^2+dy^2)/((1-|z|^2)^2) of the Poincaré hyperbolic disk.

j_n(z)=(z^n)/(2^(n+1)n!)int_0^picos(zcostheta)sin^(2n+1)thetadtheta, where j_n(z) is a spherical Bessel function of the first kind.

A smooth manifold with a Poisson bracket defined on its function space.

Let p(d) be the probability that a random walk on a d-D lattice returns to the origin. In 1921, Pólya proved that p(1)=p(2)=1, (1) but p(d)<1 (2) for d>2. Watson (1939), ...

The number q in a star polygon {p/q}.

A line segment on the boundary of a face, also called a side.