Fast on-line integer multiplication

  • Authors:
  • Michael J. Fischer;Larry J. Stockmeyer

  • Affiliations:
  • -;-

  • Venue:
  • Journal of Computer and System Sciences
  • Year:
  • 1974

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Abstract

A Turing machine multiplies binary integers on-line if it receives its inputs, low-order digit first, and produces the jth digit of the product before reading in the (j+1)st digits of the two inputs. We present a general method for converting any off-line multiplication algorithm which forms the product of two n-digit binary numbers in time F(n) into an on-line method which uses time only O(F(n) log n), assuming that F is monotone and satisfies n@?F(n)@?F(2n)/2@?kF(n) for some constant k. Applying this technique to the fast multiplication algorithm of Schonhage and Strassen gives an upper bound of O(n (log n)^2 loglog n) for on-line multiplication of integers. A refinement of the technique yields an optimal method for on-line multiplication by certain sparse integers. Other applications are to the on-line computation of products of polynomials, recognition of palindromes, and multiplication by a constant.