arithmetical progression
  算術級數	AP
  Pro·gres·sion n.
  1. The act of moving forward; a proceeding in a course; motion onward.
  2. Course; passage; lapse or process of time.
     I hope, in a short progression, you will be wholly immerged in the delices and joys of religion.   --Evelyn.
  3. Math. Regular or proportional advance in increase or decrease of numbers; continued proportion, arithmetical, geometrical, or harmonic.
  4. Mus. A regular succession of tones or chords; the movement of the parts in harmony; the order of the modulations in a piece from key to key.
  Arithmetical progression, a progression in which the terms increase or decrease by equal differences, as the numbers 
  Geometrical progression, a progression in which the terms increase or decrease by equal ratios, as the numbers 
  -- Harmonic progression, a progression in which the terms are the reciprocals of quantities in arithmetical progression, as ½, ¼, ⅙, ⅛, ¹/₁₀.
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  Ar·ith·met·ic·al a. Of or pertaining to arithmetic; according to the rules or method of arithmetic.
  Arithmetical complement of a logarithm. See Logarithm.
  Arithmetical mean. See Mean.
  Arithmetical progression. See Progression.
  Arithmetical proportion. See Proportion.
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