proof by personal communication: 'Eight-dimensional colored cycle stripping is NP-complete [Karp,

proof by personal communication:
'Eight-dimensional colored cycle stripping is NP-complete
[Karp, personal communication].'

proof by reduction to the wrong problem:
'To see that infinite-dimensional colored cycle stripping is
decidable, we reduce it to the halting problem.'

proof by reference to inaccessible literature:
The author cites a simple corollary of a theorem to be found
in a privately circulated memoir of the Slovenian
Philological Society, 1883.

proof by importance:
A large body of useful consequences all follow from the
proposition in question.
-- How to Prove It, Part 4