olemus
liik funktsioone f lähtehulgast A sihthulka B:
lähtehulga A iga kahe erineva elemendi a ja a'
kujutised f(a) ja f(a') on erinevad 
=
a function that relates a distinct element of a given set with a distinct element of another set

piltlikult
https://i.stack.imgur.com/mg8MW.jpg

https://www.mathsisfun.com/sets/images/function-mapping.gif

https://programmingcroatia.files.wordpress.com/2016/02/screen-shot-2014-08-28-at-10-40-11-am.png

ülevaateid
https://www.mathsisfun.com/sets/injective-surjective-bijective.html

https://en.wikipedia.org/wiki/Injective_function

https://math.stackexchange.com/questions/178796/what-is-the-practical-benefit-of-a-function-being-injective-surjective

vt ka
- bijektiivne funktsioon
- sürjektiivne funktsioon