## Mathematik

### Refine

#### Document Type

- Preprint (5) (remove)

#### Keywords

- Dessins d'enfants (1)
- Kochen-Specker theorem (1)
- LLL-reduction (1)
- compact Riemann surfaces (1)
- iterated subsegments (1)
- local LLL-reduction (1)
- local coordinates (1)
- segments (1)
- shortest lattice vector (1)
- subgroup growth (1)

#### Institute

- Mathematik (5)
- Informatik (2)

- Segment and strong segment LLL-reduction of lattice bases (2002)
- We present an efficient variant of LLL-reduction of lattice bases in the sense of Lenstra, Lenstra, Lov´asz [LLL82]. We organize LLL-reduction in segments of size k. Local LLL-reduction of segments is done using local coordinates of dimension 2k. Strong segment LLL-reduction yields bases of the same quality as LLL-reduction but the reduction is n-times faster for lattices of dimension n. We extend segment LLL-reduction to iterated subsegments. The resulting reduction algorithm runs in O(n3 log n) arithmetic steps for integer lattices of dimension n with basis vectors of length 2O(n), compared to O(n5) steps for LLL-reduction.

- Enhancing the security of perfect blind DL-signatures (2003)
- We enhance the security of Schnorr blind signatures against the novel one-more-forgery of Schnorr [Sc01] andWagner [W02] which is possible even if the discrete logarithm is hard to compute. We show two limitations of this attack. Firstly, replacing the group G by the s-fold direct product G exp(×s) increases the work of the attack, for a given number of signer interactions, to the s-power while increasing the work of the blind signature protocol merely by a factor s. Secondly, we bound the number of additional signatures per signer interaction that can be forged effectively. That fraction of the additional forged signatures can be made arbitrarily small.

- Conjugators of Fuchsian groups and quasiplatonic surfaces (2004)
- Let G be a Fuchsian group containing two torsion free subgroups defining isomorphic Riemann surfaces. Then these surface subgroups K and alpha-Kalpha exp(-1) are conjugate in PSl(2,R), but in general the conjugating element alpha cannot be taken in G or a finite index Fuchsian extension of G. We will show that in the case of a normal inclusion in a triangle group G these alpha can be chosen in some triangle group extending G. It turns out that the method leading to this result allows also to answer the question how many different regular dessins of the same type can exist on a given quasiplatonic Riemann surface.

- How many quasiplatonic surfaces? (2004)
- We show that the number of isomorphism classes of quasiplatonic Riemann surfaces of genus <= g has o growth of typ g exp (log g). The number of non-isomorphic regular dessins of genus <= g has the same growth type.

- Kochen-Specker theorem for von Neumann algebras (2004)
- The Kochen-Specker theorem has been discussed intensely ever since its original proof in 1967. It is one of the central no-go theorems of quantum theory, showing the non-existence of a certain kind of hidden states models. In this paper, we first offer a new, non-combinatorial proof for quantum systems with a type I_n factor as algebra of observables, including I_infinity. Afterwards, we give a proof of the Kochen-Specker theorem for an arbitrary von Neumann algebra R without summands of types I_1 and I_2, using a known result on two-valued measures on the projection lattice P(R). Some connections with presheaf formulations as proposed by Isham and Butterfield are made.