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A Proof which indirectly shows a mathematical object exists without providing a specific example or algorithm for producing an example.
See also Proof
References
Courant, R. and Robbins, H.  ``The Indirect Method of Proof.''  §2.4.4 in
  What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed.
  Oxford, England: Oxford University Press, pp. 86-87, 1996.