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 A067793 Nonprimes n such that phi(n) > 2n/3. 6
 1, 25, 35, 49, 55, 65, 77, 85, 91, 95, 115, 119, 121, 125, 133, 143, 145, 155, 161, 169, 175, 185, 187, 203, 205, 209, 215, 217, 221, 235, 245, 247, 253, 259, 265, 275, 287, 289, 295, 299, 301, 305, 319, 323, 325, 329, 335, 341, 343, 355, 361, 365, 371, 377, 391, 395, 403, 407, 413, 415, 425, 427 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Differs from A038509 in the first entry and in addition as documented in A069043. - R. J. Mathar, Sep 30 2008 It appears that a(n) lists the composite values of n which satisfy the condition sum(k^2,k=1..n) mod n = A000330(n) mod n = A215573(n) = 0. - Gary Detlefs, Nov 16 2011 Conjecture: Odd composite n such that (n^2 + 8) mod 3 = 0. (All primes > 3 meet this criterion). - Gary Detlefs, May 03 2012 Both conjectures are wrong.  The first counterexample is 385. - Robert Israel, May 17 2017 The semiprime numbers p * q, p, q > 3, are terms. - Marius A. Burtea, Oct 01 2019 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 10 is not in the list because phi(10) = 4 < 2*10/3. 25 is in the list because phi(25) = 20 > 2*25/3. MAPLE select(n -> not isprime(n) and numtheory:-phi(n) > 2*n/3, [\$1..1000]); # Robert Israel, May 17 2017 MATHEMATICA Select[Range[1000], ! PrimeQ[#] && EulerPhi[#] > 2 #/3 &] (* T. D. Noe, Nov 02 2011 *) PROG (PARI) lista(nn) = {for (n=1, nn, if (!isprime(n) && (eulerphi(n)/n > 2/3), print1(n, ", ")); ); } \\ Michel Marcus, Jul 05 2015 (MAGMA) [k:k in [1..400]| not IsPrime(k) and EulerPhi(k) gt 2*k/3]; // Marius A. Burtea, Oct 01 2019 CROSSREFS Cf. A166362. Sequence in context: A049518 A133633 A038509 * A287918 A054550 A107472 Adjacent sequences:  A067790 A067791 A067792 * A067794 A067795 A067796 KEYWORD nonn AUTHOR Benoit Cloitre, Feb 07 2002 EXTENSIONS Definition clarified by Michel Marcus, Jul 05 2015 Incorrect Maple program removed by Robert Israel, May 17 2017 STATUS approved

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Last modified January 23 13:33 EST 2020. Contains 331171 sequences. (Running on oeis4.)