Absolute value in short vectors (iteration) (thofma#1327)

* absolute value in short vectors (iteration) --------- Co-authored-by: Simon Brandhorst <[email protected]>
joschmitt · Dec 19, 2023 · fac876f · fac876f
1 parent 0900a6a
commit fac876f
Show file tree

Hide file tree

Showing 3 changed files with 52 additions and 21 deletions.
diff --git a/src/QuadForm/ShortVectors.jl b/src/QuadForm/ShortVectors.jl
@@ -4,13 +4,13 @@ export short_vectors, short_vectors_iterator, shortest_vectors, kissing_number
     short_vectors(L::ZZLat, [lb = 0], ub, [elem_type = ZZRingElem]; check::Bool = true)
                                        -> Vector{Tuple{Vector{elem_type}, QQFieldElem}}
 
-Returns all tuples `(v, n)` such that `n = v G v^t` satisfies `lb <= n <= ub`, where `G` is the
-Gram matrix of `L` and `v` is non-zero.
+Return all tuples `(v, n)` such that $n = |v G v^t|$ satisfies `lb <= n <= ub`,
+where `G` is the Gram matrix of `L` and `v` is non-zero.
 
 Note that the vectors are computed up to sign (so only one of `v` and `-v`
 appears).
 
-It is assumed and checked that `L` is positive definite.
+It is assumed and checked that `L` is definite.
 
 See also [`short_vectors_iterator`](@ref) for an iterator version.
 """
@@ -21,59 +21,71 @@ short_vectors
                            [elem_type = ZZRingElem]; check::Bool = true)
                                     -> Tuple{Vector{elem_type}, QQFieldElem} (iterator)
 
-Returns an iterator for all tuples `(v, n)` such that `n = v G v^t` satisfies
+Return an iterator for all tuples `(v, n)` such that $n = |v G v^t|$ satisfies
 `lb <= n <= ub`, where `G` is the Gram matrix of `L` and `v` is non-zero.
 
 Note that the vectors are computed up to sign (so only one of `v` and `-v`
 appears).
 
-It is assumed and checked that `L` is positive definite.
+It is assumed and checked that `L` is definite.
 
 See also [`short_vectors`](@ref).
 """
 short_vectors_iterator
 
 function short_vectors(L::ZZLat, ub, elem_type::Type{S} = ZZRingElem; check::Bool = true) where {S}
   if check
-    @req ub >= 0 "the upper bound must be non-negative"
-    @req is_definite(L) && (rank(L)==0 || gram_matrix(L)[1, 1]>0) "integer_lattice must be positive definite"
+    @req ub >= 0 "The upper bound must be non-negative"
+    @req is_definite(L) "Lattice must be definite"
   end
   if rank(L) == 0
     return Tuple{Vector{S}, QQFieldElem}[]
   end
   _G = gram_matrix(L)
+  if _G[1, 1] < 0
+    _G = -_G
+  end
   return _short_vectors_gram(Vector, _G, ub, S)
 end
 
 function short_vectors_iterator(L::ZZLat, ub, elem_type::Type{S} = ZZRingElem; check::Bool = true) where {S}
   if check
-    @req ub >= 0 "the upper bound must be non-negative"
-    @req is_definite(L) && (rank(L)==0 || gram_matrix(L)[1, 1]>0) "integer_lattice must be positive definite"
+    @req ub >= 0 "The upper bound must be non-negative"
+    @req is_definite(L) "Lattice must be definite"
   end
   _G = gram_matrix(L)
+  if rank(L) != 0 && _G[1, 1] < 0
+    _G = -_G
+  end
   return _short_vectors_gram(LatEnumCtx, _G, ub, S)
 end
 
 function short_vectors(L::ZZLat, lb, ub, elem_type::Type{S} = ZZRingElem; check=true) where {S}
   if check
-    @req lb >= 0 "the lower bound must be non-negative"
-    @req ub >= 0 "the upper bound must be non-negative"
-    @req is_definite(L) && (rank(L)==0 || gram_matrix(L)[1, 1]>0) "integer_lattice must be positive definite"
+    @req lb >= 0 "The lower bound must be non-negative"
+    @req ub >= 0 "The upper bound must be non-negative"
+    @req is_definite(L) "Lattice must be definite"
   end
   if rank(L) == 0
     return Tuple{Vector{S}, QQFieldElem}[]
   end
   _G = gram_matrix(L)
+  if _G[1, 1] < 0
+    _G = -_G
+  end
   return _short_vectors_gram(Vector, _G, lb, ub, S)
 end
 
 function short_vectors_iterator(L::ZZLat, lb, ub, elem_type::Type{S} = ZZRingElem; check=true) where {S}
   if check
-    @req lb >= 0 "the lower bound must be non-negative"
-    @req ub >= 0 "the upper bound must be non-negative"
-    @req is_definite(L) && (rank(L) == 0 || gram_matrix(L)[1, 1]>0) "integer_lattice must be positive definite"
+    @req lb >= 0 "The lower bound must be non-negative"
+    @req ub >= 0 "The upper bound must be non-negative"
+    @req is_definite(L) "Lattice must be definite"
   end
   _G = gram_matrix(L)
+  if rank(L) != 0 && _G[1, 1] < 0
+    _G = -_G
+  end
   return _short_vectors_gram(LatEnumCtx, _G, lb, ub, S)
 end
 
@@ -87,10 +99,10 @@ end
     shortest_vectors(L::ZZLat, [elem_type = ZZRingElem]; check::Bool = true)
                                                -> QQFieldElem, Vector{elem_type}, QQFieldElem}
 
-Returns the list of shortest non-zero vectors. Note that the vectors are
-computed up to sign (so only one of `v` and `-v` appears).
+Return the list of shortest non-zero vectors in absolute value. Note that the
+vectors are computed up to sign (so only one of `v` and `-v` appears).
 
-It is assumed and checked that `L` is positive definite.
+It is assumed and checked that `L` is definite.
 
 See also [`minimum`](@ref).
 """
@@ -99,9 +111,12 @@ shortest_vectors(L::ZZLat, ::ZZRingElem)
 function shortest_vectors(L::ZZLat, elem_type::Type{S} = ZZRingElem; check::Bool = true) where {S}
   if check
     @req rank(L) > 0 "Lattice must have positive rank"
-    @req is_definite(L) && (rank(L) == 0 || gram_matrix(L)[1,1]>0) "integer_lattice must be positive definite"
+    @req is_definite(L) "Lattice must be definite"
   end
   _G = gram_matrix(L)
+  if _G[1, 1] < 0
+    _G = -_G
+  end
   min, V = _shortest_vectors_gram(_G)
   L.minimum = min
   return V
@@ -116,7 +131,7 @@ end
 @doc raw"""
     minimum(L::ZZLat) -> QQFieldElem
 
-Return the minimum squared length among the non-zero vectors in `L`.
+Return the minimum absolute squared length among the non-zero vectors in `L`.
 """
 function minimum(L::ZZLat)
   @req rank(L) > 0 "Lattice must have positive rank"

diff --git a/test/QuadForm/CloseVectors.jl b/test/QuadForm/CloseVectors.jl
@@ -141,7 +141,6 @@
   @test_throws ArgumentError close_vectors(L, v, -1)
   Lm = rescale(L,-1)
   @test_throws ArgumentError close_vectors(Lm, v, 1)
-  @test_throws ArgumentError short_vectors(Lm, 1)
 
   # Test the legacy interface
 

diff --git a/test/QuadForm/ShortVectors.jl b/test/QuadForm/ShortVectors.jl
@@ -48,6 +48,9 @@
   L = integer_lattice(gram = QQ[1//2 0; 0 1//3])
   v = short_vectors(L, 0.1, 0.6)
   @test length(v) == 2
+  L = integer_lattice(gram = QQ[-1//2 0; 0 -1//3])
+  v = short_vectors(L, 0.1, 0.6)
+  @test length(v) == 2
   v = short_vectors(L, 0.4, 0.6)
   @test length(v) == 1
   @test v[1][1] == [1, 0]
@@ -74,6 +77,12 @@
   sv = @inferred short_vectors_iterator(L, delta, ZZRingElem)
   @test collect(sv) == Tuple{Vector{ZZRingElem}, QQFieldElem}[([1, 0, 0, -1], 3//10)]
 
+  L = integer_lattice(;gram = -gram)
+  sv = @inferred short_vectors_iterator(L, delta, Int)
+  @test collect(sv) == Tuple{Vector{Int64}, QQFieldElem}[([1, 0, 0, -1], 3//10)]
+  sv = @inferred short_vectors_iterator(L, delta, ZZRingElem)
+  @test collect(sv) == Tuple{Vector{ZZRingElem}, QQFieldElem}[([1, 0, 0, -1], 3//10)]
+
   L = integer_lattice(;gram = identity_matrix(ZZ, 0))
   sv = @inferred short_vectors(L, 1)
   @test collect(sv) == Tuple{Vector{ZZRingElem}, QQFieldElem}[]
@@ -89,4 +98,12 @@
   L = integer_lattice(;gram = identity_matrix(ZZ, 2))
   sv = @inferred shortest_vectors(L)
   @test length(sv) == 2
+
+  L = rescale(root_lattice(:A, 4), -1)
+  @test minimum(L) == 2
+  sv = short_vectors_iterator(L, 2, 3)
+  for (_v, n) in sv
+    v = matrix(QQ, 1, 4, _v)
+    @test (v*gram_matrix(L)*transpose(v))[1] == -n
+  end
 end