The topological support of a function #
In this file we define the topological support of a function f
, tsupport f
, as the closure of
the support of f
.
Furthermore, we say that f
has compact support if the topological support of f
is compact.
Main definitions #
Implementation Notes #
- We write all lemmas for multiplicative functions, and use
@[to_additive]
to get the more common additive versions. - We do not put the definitions in the
Function
namespace, following many other topological definitions that are in the root namespace (compareEmbedding
vsFunction.Embedding
).
The topological support of a function is the closure of its support. i.e. the closure of the set of all elements where the function is nonzero.
Equations
- tsupport f = closure (Function.support f)
Instances For
The topological support of a function is the closure of its support, i.e. the closure of the set of all elements where the function is not equal to 1.
Equations
Instances For
Functions with compact support #
A function f
has compact support or is compactly supported if the closure of
the support of f
is compact. In a T₂ space this is equivalent to f
being equal to 0
outside a
compact set.
Equations
- HasCompactSupport f = IsCompact (tsupport f)
Instances For
A function f
has compact multiplicative support or is compactly supported if the closure
of the multiplicative support of f
is compact. In a T₂ space this is equivalent to f
being equal
to 1
outside a compact set.
Equations
Instances For
Equations
- (_ : motive x) = (_ : motive x)
Instances For
Functions with compact support: algebraic operations #
If a family of functions f
has locally-finite support, subordinate to a family of
open sets, then for any point we can find a neighbourhood on which only finitely-many members of f
are non-zero.
If a family of functions f
has locally-finite multiplicative support, subordinate to a family
of open sets, then for any point we can find a neighbourhood on which only finitely-many members of
f
are not equal to 1.