TY - JOUR
T1 - Integral kašin splittings
AU - Anderson, Greg W.
PY - 2003
Y1 - 2003
N2 - For x ∈ ℝn and p ≥ 1 put ∥x∥ := (n -1 ∑ |xi|p)1/p. An orthogonal direct sum decomposition ℝ2k = E ⊕ E⊥ where dim E = k and sup0≠x∈E∪E⊥ ∥x∥ 2/∥x∥1 ≤ C called here a (k, C)-splitting. By a theorem of Kašin there exists C > 0 such that (k, C)-splittings exist for all k, and by the volume ratio method of Szarek one can take C = 32eπ. All proofs of existence of (k, C)-splittings heretofore given are nonconstructive. Here we investigate the representation of (k, C)-splittings by matrices with integral entries. For every C > 8e1/2π -1/2 and positive integer k we specify a positive integer N(k, C) such that in the set of k by 2k matrices with integral entries of absolute value not exceeding N(k, C) there exists a matrix with row span a summand in a (k, C)-splitting. We have N(k, C) ≤ 218k for k large enough depending on C. We explain in detail how to test a matrix for the property of representing a (k, C)-splitting. Taken together our results yield an explicit (if impractical) construction of (k, C)-splittings.
AB - For x ∈ ℝn and p ≥ 1 put ∥x∥ := (n -1 ∑ |xi|p)1/p. An orthogonal direct sum decomposition ℝ2k = E ⊕ E⊥ where dim E = k and sup0≠x∈E∪E⊥ ∥x∥ 2/∥x∥1 ≤ C called here a (k, C)-splitting. By a theorem of Kašin there exists C > 0 such that (k, C)-splittings exist for all k, and by the volume ratio method of Szarek one can take C = 32eπ. All proofs of existence of (k, C)-splittings heretofore given are nonconstructive. Here we investigate the representation of (k, C)-splittings by matrices with integral entries. For every C > 8e1/2π -1/2 and positive integer k we specify a positive integer N(k, C) such that in the set of k by 2k matrices with integral entries of absolute value not exceeding N(k, C) there exists a matrix with row span a summand in a (k, C)-splitting. We have N(k, C) ≤ 218k for k large enough depending on C. We explain in detail how to test a matrix for the property of representing a (k, C)-splitting. Taken together our results yield an explicit (if impractical) construction of (k, C)-splittings.
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U2 - 10.1007/BF02783423
DO - 10.1007/BF02783423
M3 - Article
AN - SCOPUS:0442275925
SN - 0021-2172
VL - 138
SP - 139
EP - 156
JO - Israel Journal of Mathematics
JF - Israel Journal of Mathematics
ER -