

A005117


Squarefree numbers: numbers that are not divisible by a square greater than 1.
(Formerly M0617)


621



1, 2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 26, 29, 30, 31, 33, 34, 35, 37, 38, 39, 41, 42, 43, 46, 47, 51, 53, 55, 57, 58, 59, 61, 62, 65, 66, 67, 69, 70, 71, 73, 74, 77, 78, 79, 82, 83, 85, 86, 87, 89, 91, 93, 94, 95, 97, 101, 102, 103, 105, 106, 107, 109, 110, 111, 113
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,2


COMMENTS

1 together with the numbers that are products of distinct primes.
Also smallest sequence with the property that a(m)*a(n) is never a square for n != m.  Ulrich Schimke (ulrschimke(AT)aol.com), Dec 12 2001
Numbers n such that there is only one Abelian group with n elements, the cyclic group of order n (the numbers such that A000688(n) = 1).  Ahmed Fares (ahmedfares(AT)mydeja.com), Apr 25 2001
Numbers n such that A007913(n) > phi(n).  Benoit Cloitre, Apr 10 2002
a(n) = smallest m with exactly n squarefree numbers <= m.  Amarnath Murthy, May 21 2002
n is squarefree <=> n divides prime(n)# where prime(n)# = product of first n prime numbers.  Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 30 2004
Numbers n such that omega(n) = Omega(n) = A072047(n).  Lekraj Beedassy, Jul 11 2006
The LCM of any finite subset is in this sequence.  Lekraj Beedassy, Jul 11 2006
This sequence and the Beatty Pi^2/6 sequence (A059535) are "incestuous": the first 20000 terms are bounded within (9, 14).  Ed Pegg Jr, Jul 22 2008
Let us introduce a function D(n) = sigma_0(n)/(2^(alpha(1) + ... + alpha(r)), sigma_0(n) number of divisors of n (A000005), prime factorization of n = p(1)^alpha(1) * ... * p(r)^alpha(r), alpha(1) + ... + alpha(r) is sequence (A086436). Function D(n) splits the set of positive integers into subsets, according to the value of D(n). Squarefree numbers (A005117) has D(n)=1, other numbers are "deviated" from the squarefree ideal and have 0<D(n)<1. For D(n)=1/2 we have A048109, for D(n)=3/4 we have A067295.  Ctibor O. Zizka, Sep 21 2008
A122840(a(n)) <= 1; A010888(a(n)) < 9.  Reinhard Zumkeller, Mar 30 2010
a(n) = A055229(A062838(n)) and a(n) > A055229(m) for m < A062838(n).  Reinhard Zumkeller, Apr 09 2010
Numbers n such that gcd(n,n')=1 where n' is the arithmetic derivative (A003415) of n.  Giorgio Balzarotti, Apr 23 2011
Numbers n such that A007913(n) = core(n) = n.  Franz Vrabec, Aug 27 2011
Numbers n such that sqrt(n) cannot be simplified.  Sean Loughran, Sep 04 2011
Indices where A057918(n)=0, i.e., positive integers m for which there are no integers k in {1,2,...,m1} such that k*m is square.  John W. Layman, Sep 08 2011
It appears that these are numbers n such that product(ithprime(k), k=1..n) mod n = 0 (see Maple code).  Gary Detlefs, Dec 07 2011.  This is the same claim as Mohammed Bouayoun's Mar 30 2004 comment above. To see why it holds: Primorial numbers, A002110, a subsequence of this sequence, are never divisible by any nonsquarefree number, A013929, and on the other hand, the index of the greatest prime dividing any n is less than n. Cf. A243291).  Antti Karttunen, Jun 03 2014
A008477(a(n)) = 1.  Reinhard Zumkeller, Feb 17 2012
A055653(a(n)) = a(n); A055654(a(n)) = 0.  Reinhard Zumkeller, Mar 11 2012
A008966(a(n)) = 1.  Reinhard Zumkeller, May 26 2012
Sum_{n>=1} 1/a(n)^s = zeta(s)/zeta(2*s).  Enrique Pérez Herrero, Jul 07 2012
A056170(a(n)) = 0.  Reinhard Zumkeller, Dec 29 2012
A013928(a(n)+1) = n.  Antti Karttunen, Jun 03 2014
Conjecture: For each n=2,3,... there are infinitely many integers b > a(n) such that Sum_{k=1..n} a(k)*b^(k1) is prime, and the smallest such an integer b does not exceed (n+3)*(n+4).  ZhiWei Sun, Mar 26 2013
The probability that a random natural number belongs to the sequence is 6/Pi^2, A059956 (see Cesàro reference).  Giorgio Balzarotti, Nov 21 2013
Booker, Hiary, & Keating give a subexponential algorithm for testing membership in this sequence without factoring.  Charles R Greathouse IV, Jan 29 2014
Because in the factorizations into prime numbers these a(n) (n >= 2) have exponents which are either 0 or 1 one could call the a(n) 'numbers with a fermionic prime number decomposition'. The levels are the prime numbers prime(j), j >= 1, and the occupation numbers (exponents) e(j) are 0 or 1 (like in Pauli's exclusion principle). A 'fermionic state' is then denoted by a sequence with entries 0 or 1, where, except for the zero sequence, trailing zeros are omitted. The zero sequence stands for a(1) = 1. For example a(5) = 6 = 2^1*3^1 is denoted by the 'fermionic state' [1, 1], a(7) = 10 by [1, 0, 1]. Compare with 'fermionic partitions' counted in A000009.  Wolfdieter Lang, May 14 2014
Numbers whose sum of divisors is equal to the sum of unitary divisors: A000203(a(n)) = A034448(a(n)).  Paolo P. Lava, Oct 08 2014
From Vladimir Shevelev, Nov 20 2014: (Start)
The following is an Eratosthenestype sieve for squarefree numbers. For integers > 1:
1) Remove even numbers, except for 2; the minimal nonremoved number is 3.
2) Replace multiples of 3 removed in step 1, and remove multiples of 3 except for 3 itself; the minimal nonremoved number is 5.
3) Replace multiples of 5 removed as a result of steps 1 and 2, and remove multiples of 5 except for 5 itself; the minimal nonremoved number is 6.
4) Replace multiples of 6 removed as a result of steps 1, 2 and 3 and remove multiples of 6 except for 6 itself; the minimal nonremoved number is 7.
5) Repeat using the last minimal nonremoved number to sieve from the recovered multiples of previous steps.
Proof. We use induction. Suppose that as a result of the algorithm, we have found all squarefree numbers less than n and no other numbers. If n is squarefree, then the number of its proper divisors d>1 is even (it is 2^k  2, where k is the number of its prime divisors), and, by the algorithm, it remains in the sequence. Otherwise, n is removed, since the number of its squarefree divisors >1 is odd (it is 2^k1).
(End)
The lexicographically least sequence of integers > 1 such that each entry has an even number of proper divisors occurring in the sequence (that's the sieve restated).  Glen Whitney, Aug 30 2015
0 is nonsquarefree because it is divisible by any square.  Jon Perry, Nov 22 2014, edited by M. F. Hasler, Aug 13 2015
The Heinz numbers of partitions with distinct parts. We define the Heinz number of a partition p = [p_1, p_2, ..., p_r] as Product_{j=1..r} prime(j) (concept used by Alois P. Heinz in A215366 as an "encoding" of a partition). For example, for the partition [1, 1, 2, 4, 10] the Heinz number is 2*2*3*7*29 = 2436. The number 30 (= 2*3*5) is in the sequence because it is the Heinz number of the partition [1,2,3].  Emeric Deutsch, May 21 2015
It is possible for 2 consecutive terms to be even; for example a(258)=422 and a(259)=426.  Thomas Ordowski, Jul 21 2015.  These form a subsequence of A077395 since their product is divisible by 4.  M. F. Hasler, Aug 13 2015
There are never more than 3 consecutive terms. Runs of 3 terms start at 1, 5, 13, 21, 29, 33, ... (A007675).  Ivan Neretin, Nov 07 2015
A046660(a(n)) = 0.  Reinhard Zumkeller, Nov 29 2015
a(n) = product of row n in A265668.  Reinhard Zumkeller, Dec 13 2015
Numbers without excess, i.e., A001221(a(n)) = A001222(a(n)).  JuriStepan Gerasimov, Sep 05 2016
Numbers n such that b^(phi(n)+1) == b (mod n) for every integer b.  Thomas Ordowski, Oct 09 2016
Boreico shows that the set of square roots of the terms of this sequence is linearly independent over the rationals.  Jason Kimberley, Nov 25 2016 (reference found by Michael Coons).


REFERENCES

I. Boreico, Linear independence of radicals, The Harvard College Mathematics Review 2(1), 8792, Spring 2008.
E. Cesàro, La serie di Lambert in aritmetica assintotica, Rend. Acc. Sc. Napoli, 1893
J.M. De Koninck, Ces nombres qui nous fascinent, Entry 165, p. 53, Ellipses, Paris 2008.
I. Niven and H. S. Zuckerman, An Introduction to the Theory of Numbers. 2nd ed., Wiley, NY, 1966, p. 251.
M. Pohst and H. Zassenhaus, Algorithmic Algebraic Number Theory, Cambridge Univ. Press, page 432.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

T. D. Noe and Daniel Forgues, Table of n, a(n) for n = 1..60794 (first 10000 terms from T. D. Noe)
Andrew R. Booker, Ghaith A. Hiary, and Jon P. Keating, Detecting squarefree numbers, arXiv:1304.6937 [math.NT], 2013.
A. Granville, ABC means we can count squarefrees, International Mathematical Research Notices 19 (1998), 9911009.
P. Haukkanen, M. Mattila, J. K. Merikoski and T. Tossavainen, Can the Arithmetic Derivative be Defined on a NonUnique Factorization Domain?, Journal of Integer Sequences, 16 (2013), #13.1.2.
A. Krowne, PlanetMath.org, squarefree number
L. Marmet, First occurrences of squarefree gaps and an algorithm for their computation
L. Marmet, First occurrences of squarefree gaps and an algorithm for their computation, arXiv preprint arXiv:1210.3829 [math.NT], 2012.
S. Ramanujan, Irregular numbers, J. Indian Math. Soc. 5 (1913) 105106.
V. Shevelev, Set of all densities of exponentially Snumbers, arXiv preprint arXiv:1511.03860 [math.NT], 2015.
Eric Weisstein's World of Mathematics, Squarefree
Wikipedia, Squarefree integer
Index entries for "core" sequences


FORMULA

Lim_{n>infinity} a(n)/n = Pi^2/6 (see A013661).  Benoit Cloitre, May 23 2002
Equals A039956 UNION A056911.  R. J. Mathar, May 16 2008
Equals A000040 UNION A006881 UNION A007304 UNION A046386 UNION A046387 UNION A067885 ...  R. J. Mathar, Nov 05 2016


MAPLE

with(numtheory); a := [ ]; for n from 1 to 200 do if issqrfree(n) then a := [ op(a), n ]; fi; od:
t:= n> product(ithprime(k), k=1..n): for n from 1 to 113 do if(t(n) mod n = 0) then print(n) fi od; # Gary Detlefs, Dec 07 2011
A005117 := proc(n) option remember; if n = 1 then 1; else for a from procname(n1)+1 do if numtheory[issqrfree](a) then return a; end if; end do: end if; end proc: # R. J. Mathar, Jan 09 2013


MATHEMATICA

Select[ Range[ 113], SquareFreeQ[ # ] &] (* Robert G. Wilson v, Jan 31 2005 *)
Select[Range[150], Max[Last /@ FactorInteger[ # ]] < 2 &] (* Joseph Biberstine (jrbibers(AT)indiana.edu), Dec 26 2006 *)
max = 0; a = {}; Do[m = FactorInteger[n]; w = Product[m[[k]][[1]], {k, 1, Length[m]}]; If[w > max, AppendTo[a, n]; max = w], {n, 1, 1000}]; a (* Artur Jasinski, Apr 06 2008 *)
NextSquareFree[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sf = n + sgn; While[c < Abs[k], While[ ! SquareFreeQ@ sf, If[sgn < 0, sf, sf++]]; If[ sgn < 0, sf, sf++]; c++]; sf + If[ sgn < 0, 1, 1]]; NestList[ NextSquareFree, 1, 70] (* Robert G. Wilson v, Apr 18 2014 *)


PROG

(MAGMA) [ n : n in [1..1000]  IsSquarefree(n) ];
(PARI) bnd = 1000; L = vector(bnd); j = 1; for (i=1, bnd, if(issquarefree(i), L[j]=i; j=j+1)); L
(PARI) {a(n)= local(m, c); if(n<=1, n==1, c=1; m=1; while( c<n, m++; if(issquarefree(m), c++)); m)} /* Michael Somos, Apr 29 2005 */
(PARI) list(n)=my(v=vectorsmall(n, i, 1), u, j); forprime(p=2, sqrtint(n), forstep(i=p^2, n, p^2, v[i]=0)); u=vector(sum(i=1, n, v[i])); for(i=1, n, if(v[i], u[j++]=i)); u \\ Charles R Greathouse IV, Jun 08 2012
(PARI) for(n=1, 113, if(core(n)==n, print1(n, ", "))); \\ Arkadiusz Wesolowski, Aug 02 2016
(Haskell)
a005117 n = a005117_list !! (n1)
a005117_list = filter ((== 1) . a008966) [1..]
 Reinhard Zumkeller, Aug 15 2011, May 10 2011


CROSSREFS

Complement of A013929. Subsequence of A072774 and A209061.
Characteristic function: A008966 (mu(n)^2, where mu = A008683).
Subsequences: A000040, A002110, A235488.
Cf. A076259 (first differences), A173143 (partial sums), A000688, A003277, A013928, A020753, A020754, A020755, A030059, A030229, A033197, A039956, A048672, A053797, A057918, A059956, A071403, A072284, A120992, A133466, A136742, A136743, A160764, A243289, A243347, A243348, A243351, A215366, A046660, A265668, A265675.
Subsequences: numbers n such that n*a(k) is squarefree where k > 1: A056911 (k = 2), A261034 (k = 3), A274546 (k = 4), A276378 (k = 5).
Sequence in context: A193304 A076144 * A144338 A077377 A076786 A167171
Adjacent sequences: A005114 A005115 A005116 * A005118 A005119 A005120


KEYWORD

nonn,easy,nice,core


AUTHOR

N. J. A. Sloane


STATUS

approved



