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Course notes (chap. 1 Number Theory, chap. 2 ... - McGill University

Course notes (chap. 1 Number Theory, chap. 2 ... - McGill University

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1.2 Efficient operations<br />

For the basic operations of +, −, ×, mod , div one may use standard “elementary<br />

school” algorithms reducing the work load by the following rules:<br />

a<br />

⎧<br />

⎨<br />

⎩<br />

+<br />

−<br />

×<br />

⎫<br />

⎬<br />

⎭ b mod n = ⎛<br />

⎝(a mod n)<br />

⎧<br />

⎨<br />

⎩<br />

+<br />

−<br />

×<br />

⎫<br />

⎬<br />

⎭ (b mod n) ⎞<br />

⎠ mod n<br />

The standard “elementary school” algorithms are precisely described in<br />

Knuth (Vol 2). For very large numbers, special purpose divide-and-conquer<br />

algorithms may be used for better efficiency of ×,mod, div. Consult the<br />

algorithmics book of Brassard-Bratley for these.

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