Indicator function
indicator (s : set α) (f : α → β) (a : α)
is f a
if a ∈ s
and is 0
otherwise.
Implementation note
In mathematics, an indicator function or a characteristic function is a function used to indicate
membership of an element in a set s
, having the value 1
for all elements of s
and the value 0
otherwise. But since it is usually used to restrict a function to a certain set s
, we let the
indicator function take the value f x
for some function f
, instead of 1
. If the usual indicator
function is needed, just set f
to be the constant function λx, 1
.
Tags
indicator, characteristic
Consider a sum of g i (f i)
over a finset
. Suppose g
is a
function such as multiplication, which maps a second argument of 0 to
- (A typical use case would be a weighted sum of
f i * h i
orf i • h i
, wheref
gives the weights that are multiplied by some other functionh
.) Then iff
is replaced by the corresponding indicator function, thefinset
may be replaced by a possibly largerfinset
without changing the value of the sum.
Summing an indicator function over a possibly larger finset
is
the same as summing the original function over the original
finset
.
Equations
- _ = _