triangular numbers

Triangular \Tri*an"gu*lar\, a. [L. triangularis: cf. F.
   triangulaire.]
   1. Having three angles; having the form of a triangle.
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   2. (Bot.) Oblong or elongated, and having three lateral
      angles; as, a triangular seed, leaf, or stem.
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   {Triangular compasses}, compasses with three legs for taking
      off the angular points of a triangle, or any three points
      at the same time.

   {Triangular crab} (Zool.), any maioid crab; -- so called
      because the carapace is usually triangular.

   {Triangular numbers} (Math.), the series of numbers formed by
      the successive sums of the terms of an arithmetical
      progression, of which the first term and the common
      difference are 1. See {Figurate numbers}, under
      {Figurate}.
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