Tahl Nowik

Unknot Diagrams Requiring a Quadratic Number of Reidemeister Moves to Untangle

Given any knot diagram E, we present a sequence of knot diagrams of the same knot type for which the minimum number of Reidemeister moves required to pass to E is quadratic with respect to the number of crossings. These bounds apply both in S2 and in ℝ2.

Differential geometry via infinitesimal displacements

We present a new formulation of some basic differential geometric notions on a smooth manifold M, in the setting of nonstandard analysis. In place of classical vector fields, for which one needs to construct the tangent bundle of M, we define a prevector field, which is an internal map from ∗ M to itself, implementing the intuitive notion of vectors as infinitesimal displacements. We introduce regularity conditions for prevector fields, defined by finite differences, thus purely combinatorial conditions involving no analysis. These conditions replace the more elaborate analytic regularity conditions appearing in previous similar approaches, e.g. by Stroyan and Luxemburg or Lutz and Goze. We define the flow of a prevector field by hyperfinite iteration of the given prevector field, in the spirit of Euler’s method. We define the Lie bracket of two prevector fields by appropriate iteration of their commutator. We study the properties of flows and Lie brackets, particularly in relation with our proposed regularity conditions. We note several simple applications to the classical setting, such as bounds related to the flow of vector fields, analysis of small oscillations of a pendulum, and an instance of Frobenius’ Theorem regarding the complete integrability of independent vector fields.

Complexity of plane and spherical curves

We show that the maximal number of singular moves required to pass between any two regularly homotopic planar or spherical curves with at most n crossings, grows quadratically with respect to n. Furthermore, this can be done with all curves along the way having at most n+2 crossings.

ON THE STRUCTURE AND AUTOMORPHISM GROUP OF FINITE ALEXANDER QUANDLES

We prove that an Alexander quandle of prime order is generated by any pair of distinct elements. Furthermore, we prove for such a quandle that any ordered pair of distinct elements can be sent to any other such pair by an automorphism of the quandle.

Finite order q-invariants of immersions of surfaces into 3-space

Abstract. Given a surface F, we are interested in

The Expected Genus of a Random Chord Diagram

{{\Bbb Z}/2}

Small oscillations of the pendulum, Euler’s method, and adequality

valued invariants of immersions of F into

The Distribution of Knots in the Petaluma Model

{{\Bbb R}^3}

COMPLEMENTARY REGIONS FOR MAPS OF SURFACES

, which are constant on each connected component of the complement of the quadruple point discriminant in

Intersection of surfaces in 3-manifolds

{Imm(F,\E)}{{\Bbb R}^3}