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 A066466 Numbers having just one anti-divisor. 6
 3, 4, 6, 96, 393216 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS See A066272 for definition of anti-divisor. Jon Perry calls these anti-primes. From Max Alekseyev, Jul 23 2007; updated May 23 2023: (Start) Except for a(2) = 4, the terms of A066466 have form 2^k*p where p is odd prime and both 2^(k+1)*p-1, 2^(k+1)*p+1 are prime (i.e., twin primes). In other words, this sequence, omitting 4, is a subsequence of A040040 containing elements of the form 2^k*p with prime p. Furthermore, since 2^(k+1)*p-1, 2^(k+1)*p+1 must equal -1 and +1 modulo 3, the number 2^(k+1)*p must be 0 modulo 3, implying that p=3. Therefore every term, except 4, must be of the form 3*2^k such that 3*2^(k+1)-1, 3*2^(k+1)+1 are twin primes. According to these tables: http://www.prothsearch.com/riesel1.html and http://www.prothsearch.com/riesel2.html there are no other such k up to 18*10^6. Therefore a(6) (if it exists) is greater than 3*2^(18*10^6) ~= 10^5418540. (End) From Daniel Forgues, Nov 23 2009: (Start) The 2 last known anti-primes seem to relate to the Fermat primes (coincidence?): 96 = 3 * 2^5 = 3 * 2^F_1 = 3 * 2^[2^(2^1) + 1] and 393216 = 3 * 2^17 = 3 * 2^F_2 = 3 * 2^[2^(2^2) + 1], where F_k is the k-th Fermat prime. (End) LINKS Table of n, a(n) for n=1..5. Jon Perry, The Anti-divisor [Cached copy] Jon Perry, The Anti-divisor: Even More Anti-Divisors [Cached copy] MATHEMATICA antid[n_] := Select[ Union[ Join[ Select[ Divisors[2n - 1], OddQ[ # ] && # != 1 & ], Select[ Divisors[2n + 1], OddQ[ # ] && # != 1 & ], 2n/Select[ Divisors[ 2n], OddQ[ # ] && # != 1 &]]], # < n & ]]; Select[ Range[10^5], Length[ antid[ # ]] == 1 & ] CROSSREFS Cf. A066272. A066272(a(n)) = 1. Cf. A000215, A019434. - Daniel Forgues, Nov 23 2009 Sequence in context: A256326 A239244 A267943 * A332511 A129293 A233512 Adjacent sequences: A066463 A066464 A066465 * A066467 A066468 A066469 KEYWORD nonn,hard AUTHOR Robert G. Wilson v, Jan 02 2002 EXTENSIONS Edited by Max Alekseyev, Oct 13 2009 STATUS approved

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Last modified September 11 06:30 EDT 2024. Contains 375814 sequences. (Running on oeis4.)