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 A033950 Refactorable numbers: number of divisors of n divides n. Also known as tau numbers. 157
 1, 2, 8, 9, 12, 18, 24, 36, 40, 56, 60, 72, 80, 84, 88, 96, 104, 108, 128, 132, 136, 152, 156, 180, 184, 204, 225, 228, 232, 240, 248, 252, 276, 288, 296, 328, 344, 348, 360, 372, 376, 384, 396, 424, 441, 444, 448, 450, 468, 472, 480, 488, 492, 504, 516, 536 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Kennedy and Cooper show that this sequence has density zero. Spiro showed more precisely that the number of refactorable numbers less than x is asymptotic to (x/sqrt(log x))(log(log x))^(-1+o(1)). - David Eppstein, Aug 25 2014 Numbers n such that the equation gcd(n,x)=tau(n) has solutions. - Benoit Cloitre, Jun 10 2002 Refactorable numbers are the fixed points of A009230. - Labos Elemer, Nov 18 2002 Let ref(n) denote the characteristic function of the refactorable numbers. Then ref(n) = 1 + floor(n/d(n)) - ceiling(n/d(n)), where d(n) is the number of divisors of n. - Wesley Ivan Hurt, Jan 09 2013, Feb 15 2013 An odd number with an even number of divisors cannot be in the sequence by definition. Therefore all odd terms are squares (A000290). - Ivan N. Ianakiev, Aug 25 2013 A054008(n) = n mod A000005(n). - Reinhard Zumkeller, Sep 17 2014 The only squarefree terms are 1 and 2: if x is a squarefree number that is a product of n distinct primes, its number of divisors is 2^n, so x is refactorable if it contains 2^n as a factor, but that makes it nonsquarefree unless n = 0, 1, hence x = 1, 2.  - Waldemar Puszkarz, Jun 10 2016 Every positive integer occurs as tau(n) for some n in the sequence. If the factorization of n is Product p_i^k_i, then Product p_i^(p_i^k_i-1) has the specified property. For n prime, this is the only such number. - Franklin T. Adams-Watters, Jan 14 2017 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, B12. New Scientist, Sep 05 1998, p. 17, para. 3. LINKS T. D. Noe and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe) Kushagr Ahuja, Patrick Lei, Dylan Pentland, Tau ideals in number fields, PROMYS 2017. A. Bundy, Simon Colton, T. Walsh, HR - A system for Machine Discovery in Finite Algebras, ECAI 1998. S. Colton, Refactorable Numbers - A Machine Invention, J. Integer Sequences, Vol. 2, 1999, #2. Robert E. Kennedy and Curtis N. Cooper, Tau numbers, natural density and Hardy and Wright's Theorem 437, International Journal of Mathematics and Mathematical Sciences, 13:2 (1990), pp. 383-386. Claudia Spiro, How often is the number of divisors of n a divisor of n?, J. Number Theory 21 (1985), no. 1, 81-100. Joshua Zelinsky, Tau Numbers: A Partial Proof of a Conjecture and Other Results , Journal of Integer Sequences, Vol. 5 (2002), Article 02.2.8. MAPLE with(numtheory): A033950 := proc(n)     option remember:     local k:     if n=1 then         return 1:     else         for k from procname(n-1)+1 do             if type(k/tau(k), integer) then                 return k:             end if:         end do:     end if: end proc: seq(A033950(n), n=1..56); # Nathaniel Johnston, May 04 2011 MATHEMATICA Do[If[IntegerQ[n/DivisorSigma[0, n]], Print[n]], {n, 1, 1000}] Select[ Range, Mod[ #, DivisorSigma[0, # ]] == 0 &] Select[Range, Divisible[ #, DivisorSigma[0, # ]]&] (* Waldemar Puszkarz, Jun 10 2016 *) PROG (MAGMA) [ n: n in [1..540] | n mod #Divisors(n) eq 0 ]; // Klaus Brockhaus, Apr 29 2009 (PARI) isA033950(n)=n%numdiv(n)==0 \\ Charles R Greathouse IV, Jun 10 2011 (Haskell) a033950 n = a033950_list !! (n-1) a033950_list = [x | x <- [1..], x `mod` a000005 x == 0] -- Reinhard Zumkeller, Dec 28 2011 (Python) from sympy import divisor_count print([n for n in range(1, 1001) if not n % divisor_count(n)]) # Indranil Ghosh, May 03 2017 CROSSREFS Cf. A000005, A039819, A036762, A051278, A051279, A051280, A036763. Cf. A235353 (subsequence). Cf. A054008, A281188. Sequence in context: A294374 A066550 A162952 * A046526 A279373 A057529 Adjacent sequences:  A033947 A033948 A033949 * A033951 A033952 A033953 KEYWORD nonn,nice AUTHOR Simon Colton (simonco(AT)cs.york.ac.uk) EXTENSIONS More terms from Erich Friedman STATUS approved

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Last modified September 19 01:17 EDT 2020. Contains 337175 sequences. (Running on oeis4.)