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==duplicate of addition-chain exponentiation?==

This looks like a duplicate of [[Addition-chain exponentiation]]. Perhaps some of the content here belongs on that page? --[[User:Rdm|Rdm]] ([[User talk:Rdm|talk]]) 11:46, 2 June 2015 (UTC)

''Knuth's power tree'' isn't a duplicate of ''addition-chain exponentiation'', Knuth's power tree is much simpler to compute, and in almost all of the exponentiation process, is different than addition-chain exponentiation (but yields the same result, of course, albeit in a different way).

In the Rosetta Code task ''Addition-chain exponentiation'', there's a lot of reference to the ''binary'' method (also known elsewhere as ''the factor method'') which, as noted in the preamble text of this Rosetta Code task:

:: For ''n'' ≤ 100,000, the power tree method: ::::* bests the factor method 88,803 times, ::::* ties 11,191 times, ::::* loses 6 times.

From this, it can be seen that Knuth's power tree isn't always the best algorithm for exponentiation (as compared to ''the factor method''), but it's better over 88% of the time.

Knuth's power tree probably resembles addition-chain exponentiation as much as calculation of primes via ''trial division'' versus ''Sieve of Eratosthenes'', they have the same result, but with different strategies and complexity. -- [[User:Gerard Schildberger|Gerard Schildberger]] ([[User talk:Gerard Schildberger|talk]]) 17:49, 2 June 2015 (UTC)

: Ah, my mistake.

: Triggered, on re-reading, by an utter lack of description of the algorithm on the page itself, combined with [for me] unfamiliar (and unlinked) terms for familiar concepts (like "factor method" - technically all of these approaches are "factor methods" so without some definition the term winds up being ambiguous). And I don't have a copy of Knuth's book handy. I guess I am supposed to reverse engineer one of the linked implementations? --[[User:Rdm|Rdm]] ([[User talk:Rdm|talk]]) 18:13, 2 June 2015 (UTC)

:: I have Knuth's Vol. 2 book, but the algorithm is way beyond my ability to explain it or paraphrase it, or even parrot it. (My utter is bigger than your utter, and I have a book for help). Even if I could explain it, Dr. Knuth's explanation of pretty detailed and extensive. I've read (and re-read) the particular chapter and still don't know it well enough to explain it. I was thinking about quoting the text wholesale, but I didn't want to push that particular envelope. Perhaps someone with a much better mathematics background could dive in that pool and illuminate the process (well, Dr. Knuth did, but I'm still a bit mystified and unilluminated). I was hoping that the ~~ two ~~ several computer programming examples (as referenced by the links) would provide enough insight to code other computer programming language examples (entries). -- [[User:Gerard Schildberger|Gerard Schildberger]] ([[User talk:Gerard Schildberger|talk]]) 19:33, 2 June 2015 (UTC)

The ''factor method'' or ''binary factor method'' or more simply ''binary method'', is as follows (as mentioned in Knuth's book):

(Note that this isn't a description of Knuth's power tree method)

For a positive integer ''n'' (for an exponent): ::* express ''n'' in binary form (ignore any leading zeroes) ::* substitute each 1 by the (two) letters SX ::* substitute each 0 by the (single) letter S ::* remove the (two) leading (on the left) letters SX

We now have a method (or "rule") for computing x^{n}.

(From left to right):

::* S means to ''square'' the number, ::* X means to ''multiply'' the number by x.

''' ══════════ An illustrative example of the binary factor method ══════════ '''

for the power 23,

it's binary representation is 10111,

so the following sequence is: SX S SX SX SX.

Now, remove the leading (left) SX two letters,

resulting in S SX SX SX.

So the rule (sequence) is: :::* square :::* square :::* multiple by x :::* square :::* multiple by x :::* square :::* multiple by x

Or, in wording it in another way, we have computed:
::::* x^{2}
::::* x^{4}
::::* x^{5}
::::* x^{10}
::::* x^{11}
::::* x^{22}
::::* x^{23}

-- [[User:Gerard Schildberger|Gerard Schildberger]] ([[User talk:Gerard Schildberger|talk]]) 19:33, 2 June 2015 (UTC)

:: Yes, but it's the power tree algorithm which we need here. Actually, I found https://comeoncodeon.wordpress.com/2009/03/02/evaluation-of-powers/ which indicates that the factor method is different from the binary method. I'm studying its description of the power tree algorithm now. --[[User:Rdm|Rdm]] ([[User talk:Rdm|talk]]) 21:59, 2 June 2015 (UTC)