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Proving primality in essentially quartic random time. (English) Zbl 1144.11085

In July 2002 appeared online the preprint “Primes is in P”, by Manindra Agrawal, Neeraj Kayal, and Nitin Saxena which describes a deterministic polynomial time algorithm (henceforth referred to as “AKS”) to determine the primality of an integer \(n\). The appearance of such an algorithm was totally unexpected and unleashed a feeding frenzy among the primality testing community, as they searched for improvements. The essence of AKS is succinctly described in the present paper as “combinatorics in a cyclotomic extension of \({\mathbb Z}/(n)\)” and the first improvements came mainly from fine-tuning the combinatorics, though also from sharpening bounds on quantities used the the algorithm. Many of these improvements were incorporated in the published version [M. Agrawal, N. Kayal and N. Saxena, Ann. Math. (2) 160, No. 2, 781–793 (2004; Zbl 1071.11070)]. The published algorithm has complexity \(\widetilde{O}(\log n^{7.5})\), whereas in the preprint the complexity is only proved to be \(\widetilde{O}(\log n^{12})\) where the notation \(\widetilde O(m)\) means \(O(m\) polynomial in \(\log m)\)).
A fundamental advance was made by P. Berrizbeitia [Math. Comput. 74, 2043–2059 (2005; Zbl 1071.11071)]. The preprint appeared in 2003, who showed that, for \(n\) such that \(n^2-1\) is divisible by a power of 2 close to \((\log n)^2\), one can do the combinatorics in a Kummer, rather than cyclotomic, extension of \({\mathbb Z}/(n)\), and the resulting algorithm is \(\widetilde{O}(\log n^{4})\). Berrizbeitia’s ideas were taken up by Q. Cheng [Primality Proving via One Round in ECPP and One Iteration in AKS, to appear in J. Cryptology, Preprint at http://www.cs.ou.edu/~qchengpub.html], Mihailescu and Avanzi (unpublished) and Bernstein in the present paper. Chen showed how to apply Berrizbeitia’s algorithm when one has any prime divisor of \(n-1\) close to \((\log n)^2\). Bernstein extends the algorithm to the case that one has an arbitrary divisor \(e \approx (\log n)^2\) of \(n^d-1\), where \(d \in n^{o(1)}\). The existence of a suitable pair \((d,e)\) for any prime \(n\) is proved in the paper, as a consequence of a theorem of Odlyzko and Pomerance. The resulting algorithm runs in time \((\log n^{4+o(1)})\). The basic theorem is:
Let \(n,d\) and \(e\) be positive integers such that \(2^e-1 \geq n^{2d\lfloor\sqrt{e}\rfloor}\) and \(e\) divides \(n^d-1\). Let \(f\) be a monic polynomial in \({\mathbb Z}/(n)[y]\) of degree \(d\). Define \(R\) as the ring \({\mathbb Z}/(n)[y]/f\). Let \(r\) be an element of \(R\) such that \(r^{n^d-1} =1\) in \(R\), \(r^{(n^d-1)/q}-1\) is a unit in \(R\) for each prime \(q\) dividing \(e\), and \(r-1\) is a unit in \(R\). If \((x-1)^{n^d}=r^{(n^d-1)/e}x-1\) in the ring \(R[x]/(x^e-r)\) then \(n\) is a power of a prime.
In fact a slightly refined version of the theorem is proved. Careful estimates of constant factors are given, and issues relevant to implementation discussed. Also included is a section on the relation between the work of Mihailescu and Avanzi and the current paper. It is still problematic as to whether the algorithm is truly practical, and competitive with (hyper)elliptic curve algorithms or, indeed, the subexponential algorithm usually known as APRCL [see H. Cohen and H. W. Lenstra, jun., Math. Comput. 42, 297–330 (1984; Zbl 0578.10004)] which in practice is extremely efficient. In this respect the author conjectures that, in his algorithm, \(d=1\) is the only relevant case, since if \(d>1\) one can follow Chen (loc. cit.) and use an elliptic curve step to replace \(n\) by an auxiliary \(n^\prime\) that-one hopes- has suitable divisors of \(n^\prime -1\). Conjecturally, the elliptic curve step takes time \(O((\log n)^{3+o(1)}\), so is worthwhile – even if it has to be repeated several times in order to reduce to \(d=1\). Experiment seems to confirm this, but nothing is proved. Finally, it should be noted that all the AKS-type algorithms in the Berrizbeitia line introduce a random element. To the reviewer’s knowledge, the only strictly deterministic polynomial time primality test is the original, cyclotomic version of AKS.

MSC:

11Y11 Primality

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gmp
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