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In mathematics, an addition chain for computing a positive integer n can be given by a sequence of natural numbers v and a sequence of index pairs w such that each term in v is the sum of two previous terms, the indices of those terms being specified by w:

v =(v0,...,vs), with v0 = 1 and vs = n
for each 0< is holds: vi = vj + vk, with wi=(j,k) and 0 ≤ j,ki − 1

Often only v is given since it is easy to extract w from v, but sometimes w is not uniquely reconstructible. An introduction is given by Knuth.

Examples

As an example: v = (1,2,3,6,12,24,30,31) is an addition chain for 31 of length 7, since

2 = 1 + 1
3 = 2 + 1
6 = 3 + 3
12 = 6 + 6
24 = 12 + 12
30 = 24 + 6
31 = 30 + 1

Addition chains can be used for addition-chain exponentiation: so for example we only need 7 multiplications to calculate 531:

52 = 51 × 51
53 = 52 × 51
56 = 53 × 53
512 = 56 × 56
524 = 512 × 512
530 = 524 × 56
531 = 530 × 51

Calculating an addition chain of minimal length is not easy; a generalized version of the problem, in which one must find a chain that simultaneously forms each of a sequence of values, is NP-complete. There is no known algorithm which can calculate a minimal addition chain for a given number with any guarantees of reasonable timing or small memory usage. However, several techniques to calculate relatively short chains exist. One very well known technique to calculate relatively short addition chains is the binary method, similar to exponentiation by squaring. Other well-known methods are the factor method and window method.

Chain length

Let l(n) denote the smallest s so that there exists an addition chain of length s which computes n. It is known that 

$$\log_2(n)+ \log_2(\nu(n))-2.13\leq l(n) \leq \log_2(n) + \log_2(n)(1+o(1))/\log_2(\log_2(n)),$$

where $$\nu(n)$$ is Hamming weight of binary expansion of n.

It is clear that l(2n) ≤ l(n)+1. Strict inequality is possible, as l(382) = l(191) = 11, observed by Knuth. The first integer with l(2n) < l(n) is n = 375494703.
Brauer chain

A Brauer chain or star addition chain is an addition chain in which one of the summands is always the previous chain: that is,

for each k>0: ak = ak-1 + aj for some j < k.

A Brauer number is one for which the Brauer chain is minimal.

Brauer proved that

l(2n  − 1) ≤ n − 1 + l(n) .

where l* is the length of the shortest star chain. For many values of n,and in particular for n ≤ 2500, they are equal: l(n) = l*(n). But Hansen showed that there are some values of n for which l(n) ≠ l*(n), such as n = 26106 + 23048 + 22032 + 22016 + 1 which has l*(n) = 6110, l(n) ≤ 6109.

Scholz conjecture
Main article: Scholz conjecture

The Scholz conjecture (sometimes called the Scholz–Brauer or Brauer–Scholz conjecture), named after A. Scholz and Alfred T. Brauer), is a conjecture from 1937 stating that

l(2n − 1) ≤ n − 1 + l(n) .

It is known to be true for Hansen numbers, a generalization of Brauer numbers; N. Clift checked by computer that all n≤5784688 are Hansen (while 5784689 is not). Clift further checked that is true with equality for n≤64.

Lucas chain

References

D. E. Knuth, The Art of Computer Programming, Vol 2, "Seminumerical Algorithms", Section 4.6.3, 3rd edition, 1997
Downey, Peter; Leong, Benton; Sethi, Ravi (1981). "Computing sequences with addition chains". SIAM Journal on Computing 10 (3): 638–646. doi:10.1137/0210047.. A number of other papers state that finding a single addition chain is NP-complete, citing this paper, but it does not claim or prove such a result.
A. Schönhage A lower bound on the length of addition chains, Theoret. Comput. Sci. 1 (1975), 1–12.
Guy (2004) p.169

Clift, Neill Michael (2011). "Calculating optimal addition chains" (PDF). Computing 91 (3): 265–284. doi:10.1007/s00607-010-0118-8.

Brauer, Alfred (1939). "On addition chains". Bulletin of the American Mathematical Society 45 (10): 736–739. doi:10.1090/S0002-9904-1939-07068-7. ISSN 0002-9904. MR 0000245.
Richard K. Guy (2004). Unsolved Problems in Number Theory. Springer-Verlag. ISBN 0-387-20860-7. OCLC 54611248. Zbl 1058.11001. Section C6.

"Sloane's A003313 : Length of shortest addition chain for n", The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
F. Bergeron, J. Berstel. S. Brlek "Efficient computation of addition chains"

1 Examples of spectral methods
1.1 A concrete, linear example
1.1.1 Algorithm
1.2 A concrete, nonlinear example
2 A relationship with the spectral element method