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 A033181 Absolute Euler pseudoprimes: odd composite numbers n such that a^((n-1)/2) == +-1 (mod n) for every a coprime to n. 8
 1729, 2465, 15841, 41041, 46657, 75361, 162401, 172081, 399001, 449065, 488881, 530881, 656601, 670033, 838201, 997633, 1050985, 1615681, 1773289, 1857241, 2113921, 2433601, 2455921, 2704801, 3057601, 3224065, 3581761, 3664585, 3828001, 4463641, 4903921 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS These numbers n have the property that, for each prime divisor p, p-1 divides (n-1)/2. E.g., 2465 = 5*17*29; 1232/4 = 308; 1232/16 = 77; 1232/28 = 44. - Karsten Meyer, Nov 08 2005 All these numbers are Carmichael numbers (A002997). - Daniel Lignon, Sep 12 2015 These are odd composite numbers n such that b^((n-1)/2) == 1 (mod n) for every base b that is a quadratic residue modulo n and coprime to n. There are no odd composite numbers n such that b^((n-1)/2) == -1 (mod n) for every base b that is a quadratic non-residue modulo n and coprime to n. Note: the absolute Euler-Jacobi pseudoprimes do not exist. Theorem: if an absolute Euler pseudoprime n is a Proth number, then b^((n-1)/2) == 1 for every b coprime to n; by Proth's theorem. Such numbers are 1729, 8355841, 40280065, 53282340865, ...; for example, 1729 = 27*2^6 + 1 with 27 < 2^6. However, it seems that all absolute Euler pseudoprimes n satisfy the stronger congruence b^((n-1)/2) == 1 (mod n) for every b coprime to n, as evidenced by the modified Korselt's criterion (see the first comment). It should be noted that these are odd numbers n such that Carmichael's lambda(n) divides (n-1)/2. Also, these are odd numbers n > 1 coprime to Sum_{k=1..n-1} k^{(n-1)/2}. - Amiram Eldar and Thomas Ordowski, Apr 29 2019 Carmichael numbers k such that (p-1)|(k-1)/2 for each prime p|k. These are odd composite numbers k with half (the maximal possible fraction) of the numbers 1 <= b < k coprime to k that are bases in which k is an Euler-Jacobi pseudoprime, i.e. A329726((k-1)/2)/phi(k) = 1/2. - Amiram Eldar, Nov 20 2019 LINKS Daniel Lignon and Dana Jacobsen, Table of n, a(n) for n = 1..10000 (first 124 terms from Daniel Lignon) Lorenzo Di Biagio, Euler Pseudoprimes for Half of the Bases, Integers, Vol. 12, No. 6 (2012), pp. 1231-1237, arXiv preprint, arXiv:1109.3596 [math.NT] (2011). Math Help Forum, how many absolute euler pseudoprimes less than a million, Sep 2009. Louis Monier, Evaluation and comparison of two efficient primality testing algorithms, Theoretical Computer Science, Vol. 11 (1980), pp. 97-108. FORMULA a(n) == 1 (mod 4). - Thomas Ordowski, May 02 2019 MAPLE filter:=  proc(n)   local q;   if isprime(n) then return false fi;   if 2 &^ (n-1) mod n <> 1 then return false fi;   if not numtheory:-issqrfree(n) then return false fi;   for q in numtheory:-factorset(n) do     if (n-1)/2 mod (q-1) <> 0 then return false fi   od:   true; end proc: select(filter, [seq(i, i=3..10^7, 2)]); # Robert Israel, Nov 24 2015 MATHEMATICA absEulerpspQ[n_Integer?PrimeQ]:=False; absEulerpspQ[n_Integer?EvenQ]:=False; absEulerpspQ[n_Integer?OddQ]:=Module[{a=2}, While[a 1 && Max[f[[;; , 2]]]==1 && AllTrue[p, Divisible[(n-1)/2, #-1] &]]; Select[Range[3, 2*10^5], aQ] (* Amiram Eldar, Nov 20 2019 *) PROG (Perl) use ntheory ":all"; my \$n; foroddcomposites { say if is_carmichael(\$_) && vecall { ((\$n-1)>>1) % (\$_-1) == 0 } factor(\$n=\$_); } 1e6; # Dana Jacobsen, Dec 27 2015 CROSSREFS Cf. A002997, A006970, A047713, A080075, A329726. Sequence in context: A154717 A306478 A051388 * A300949 A198775 A154729 Adjacent sequences:  A033178 A033179 A033180 * A033182 A033183 A033184 KEYWORD nonn AUTHOR N. J. A. Sloane, Dec 11 1999 EXTENSIONS "Absolute Euler pseudoprimes" added to name by Daniel Lignon, Sep 09 2015 Definition edited by Thomas Ordowski, Apr 29 2019 STATUS approved

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Last modified October 25 15:37 EDT 2020. Contains 338012 sequences. (Running on oeis4.)