arXiv subjects

Tobias Ekholm

Publications and source records attributed to Tobias Ekholm.

60 records · Page 4Linked to original sources

arXiv2002

Framed holonomic knots

A holonomic knot is a knot in 3-space which arises as the 2-jet extension of a smooth function on the circle. A holonomic knot associated to a generic function is naturally framed by the blackboard framing of the knot diagram associated to the 1-jet extension of the function. There are two classical invariants of framed knot diagrams: the Whitney index (rotation number) W and the self linking number S. For a framed holonomic knot we show that W is bounded above by the negative of the braid index of the knot, and that the sum of W and |S| is bounded by the negative of the Euler characteristic of any Seifert surface of the knot. The invariant S restricted to framed holonomic knots with W=m, is proved to split into n, where n is the largest natural number with 2n < |m|+1, integer invariants. Using this, the framed holonomic isotopy classification of framed holonomic knots is shown to be more refined than the regular isotopy classification of their diagrams.

math.GT↗

arXiv2001

The complex shade of a real space and its applications

A natural oriented (2k+2)-chain in CP^{2k+1} with boundary twice RP^{2k+1}, its complex shade, is constructed. Via intersection numbers with the shade, a new invariant, the shade number of k-dimensional subvarieties with normal vector fields along their real part, is introduced. For an even-dimensional real variety, the shade number and the Euler number of the complement of the normal vector field in the real normal bundle of its real part agree. For an odd- dimensional orientable real variety, a linear combination of the shade number and the wrapping number (self-linking number) of its real part is independent of the normal vector field and equals the encomplexed writhe as defined by Viro. Shade numbers of varieties without real points and encomplexed writhes of odd- dimensional real varieties are, in a sense, Vassiliev invariants of degree 1. Complex shades of odd-dimensional spheres are constructed. Shade numbers of real subvarieties in spheres have properties analogous to those of their projective counterparts.

math.GT↗

arXiv2000

Geometric formulas for Smale invariants of codimension two immersions

We give three formulas expressing the Smale invariant of an immersion f of a (4k-1)-sphere into (4k+1)-space. The terms of the formulas are geometric characteristics of any generic smooth map g of any oriented 4k-dimensional manifold, where g restricted to the boundary is an immersion regularly homotopic to f in (6k-1)-space. The formulas imply that if f and g are two non-regularly homotopic immersions of a (4k-1)-sphere into (4k+1)-space then they are also non-regularly homotopic as immersions into (6k-1)-space. Moreover, any generic homotopy in (6k-1)-space connecting f to g must have at least a_k(2k-1)! cusps, where a_k=2 if k is odd and a_k=1 if k is even.

math.GT↗

arXiv2000

Differential 3-knots in 5-space with and without self intersections

Regular homotopy classes of immersions of a 3-sphere in 5-space constitute an infinite cyclic group. The classes containing embeddings form a subgroup of index 24. The obstruction for a generic immersion to be regularly homotopic to an embedding is described in terms of geometric invariants of its self intersection. Geometric properties of self intersections are used to construct two invariants J and St of generic immersions which are analogous to Arnold's invariants of plane curves. We prove that J and St are independent first order invariants and that any first order invariant is a linear combination of these. As by-products, some invariants of immersions of 3-spheres in 4-space are obtained. Using them, we find restrictions on the topology of self intersections.

math.GT↗

arXiv1999

Total curvatures of holonomic links

A differential geometric characterization of the braid-index of a link is found. After multiplication by 2pi, it equals the infimum of the sum of total curvature and total absolute torsion over holonomic representatives of the link. Upper and lower bounds for the infimum of total curvature over holonomic representatives of a link are given in terms of its braid- and bridge-index. Examples showing that these bounds are sharp are constructed.

math.GT↗

arXiv1999

Invariants of generic immersions and Bernoulli numbers

First order invariants of generic immersions of manifolds of dimension nm-1 into manifolds of dimension n(m+1)-1, m,n>1 are constructed using the geometry of self-intersections. The range of one of these invariants is related to Bernoulli numbers. As by-products some geometrically defined invariants of regular homotopy are found.

math.GT↗