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 A059269 Numbers m for which the number of divisors, tau(m), is divisible by 3. 11
 4, 9, 12, 18, 20, 25, 28, 32, 36, 44, 45, 49, 50, 52, 60, 63, 68, 72, 75, 76, 84, 90, 92, 96, 98, 99, 100, 108, 116, 117, 121, 124, 126, 132, 140, 144, 147, 148, 150, 153, 156, 160, 164, 169, 171, 172, 175, 180, 188, 196, 198, 200, 204, 207, 212, 220, 224, 225, 228 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS tau(n) is divisible by 3 iff at least one prime in the prime factorization of n has exponent of the form 3*m + 2. This sequence is an extension of the sequence A038109 in which the numbers has at least one prime with exponent 2 (the case of m = 0 here ) in their prime factorization. The union of A211337 and A211338 is the complementary sequence to this one. - Douglas Latimer, Apr 12 2012 Numbers whose cubefree part (A050985) is not squarefree (A005117). - Amiram Eldar, Mar 09 2021 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 Eckford Cohen, Arithmetical Notes, XIII. A Sequal to Note IV, Elemente der Mathematik, Vol. 18 (1963), pp. 8-11. S. S. Pillai, On a congruence property of the divisor function, J. Indian Math. Soc. (N. S.), Vol. 6, (1942), pp. 118-119. L. G. Sathe, On a congruence property of the divisor function, American Journal of Mathematics, Vol. 67, No. 3 (1945), pp. 397-406. FORMULA Conjecture: a(n) ~ k*n where k = 1/(1 - Product(1 - (p-1)/(p^(3*i)))) = 3.743455... where p ranges over the primes and i ranges over the positive integers. - Charles R Greathouse IV, Apr 13 2012 The asymptotic density of this sequence is 1 - zeta(3)/zeta(2) = 1 - 6*zeta(3)/Pi^2 = 0.2692370305... (Sathe, 1945). Therefore, the above conjecture, a(n) ~ k*n, is true, but k = 1/(1-6*zeta(3)/Pi^2) = 3.7141993349... - Amiram Eldar, Jul 26 2020 EXAMPLE a(7) = 28 is a term because the number of divisors of 28, d(28) = 6, is divisible by 3. MAPLE with(numtheory): for n from 1 to 1000 do if tau(n) mod 3 = 0 then printf(`%d, `, n) fi: od: MATHEMATICA Select[Range, Divisible[DivisorSigma[0, #], 3] &] (* Amiram Eldar, Jul 26 2020 *) PROG (PARI) is(n)=vecmax(factor(n)[, 2]%3)==2 \\ Charles R Greathouse IV, Apr 10 2012 (PARI) is(n)=numdiv(n)%3==0 \\ Charles R Greathouse IV, Sep 18 2015 CROSSREFS Cf. A000005, A005117, A038109, A050985, A211337, A211338, A253905. Sequence in context: A344275 A312862 A177880 * A081619 A336594 A304365 Adjacent sequences:  A059266 A059267 A059268 * A059270 A059271 A059272 KEYWORD nonn,easy AUTHOR Avi Peretz (njk(AT)netvision.net.il), Jan 24 2001 EXTENSIONS More terms from James A. Sellers, Jan 24 2001 STATUS approved

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Last modified December 3 10:34 EST 2021. Contains 349462 sequences. (Running on oeis4.)