def inner_prod(a, b)
      return (0..2).inject(0.0) { |s, i| s + a[i] * b[i] }
    rescue => e
      raise
    end
def inner_prod(a, b)
      return (0..2).inject(0.0) { |s, i| s + a[i] * b[i] }
    rescue => e
      raise
    end
=========================================================================
 ベクトルの内積

 @param:  a  (ベクトル)
 @param:  b  (ベクトル)
 @return: w  (スカラー(内積の値))
=========================================================================