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Differential forms (co-vectors) are functions (elements of dual vector-space) which map vectors to real numbers.
P-forms are important, because they are exactly the objects that we need if we want to talk about areas and volumes (and higher dimensional analogues).
http://jakobschwichtenberg.com/vectors-forms-p-vectors-p-forms-and-tensors/
‘Hamiltonian mechanics cannot be understood without differential forms’.
Mathematical methods of classical mechanics by Wladimir Igorewitsch Arnold, p. 163