0,3

Number of elements in the set {(x,y): 1<=x<=y<=n, 1=gcd(x,y)}.

Sum_{k=1..n} phi(k) gives the number of distinct arithmetic progressions which contain infinite number of primes and whose difference does not exceed n. E.g. {1k+1}, {2k+1}, {3k+1, 3k+2}, {4k+1, 4k+3}, {5k+1, ..5k+4} means 10 sequences. - Labos E. (labos(AT)ana.sote.hu), May 02 2001

The quotient A024916[n]/a[n] = SummatorySigma/SummatoryTotient as n increases seems to approach Pi^4/36 or zeta(2)^2 [~2.705808084277845]. - Labos E. (labos(AT)ana.sote.hu), Sep 20 2004. Corrected by Peter Pein, Apr 28 2009.

Also the number of rationals p/q in (0,1] with denominators q<=n. - Franz Vrabec (franz.vrabec(AT)aon.at), Jan 29 2005

a(n) is the number of initial segments of Beatty sequences for real numbers > 1, cut off when the next term in the sequence would be >= n. For example, the sequence 1,2 is included for n=3 and n=4, but not for n>=5 because the next term of the Beatty sequence must be 3 or 4. Problem suggested by David W. Wilson. - Franklin T. Adams-Watters (FrankTAW(AT)Netscape.net), Oct 19 2006

Number of complex numbers satisfying any one of {x^1=1, x^2=1, x^3=1, x^4=1, x^5=1, ..., x^n=1}. - Paul Smith (math.idiot(AT)gmail.com), Mar 19 2007

A. Beiler, Recreations in the Theory of Numbers, Dover, 1966, Chap. XVI.

S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 115-119.

R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics. Addison-Wesley, Reading, MA, 1990, p. 138.

M. N. Huxley, The Distribution of Prime Numbers, Oxford Univ. Press, 1972, p. 6.

D. H. Lehmer, Guide to Tables in the Theory of Numbers. Bulletin No. 105, National Research Council, Washington, DC, 1941, pp. 7-10.

Paul Loomis, Michael Plytage and John Polhill, Summing up the Euler 'phi' function, The College Mathematics Journal, vol. 39 (#1), pp. 34-42.

D. S. Mitrinovic et al., Handbook of Number Theory, Kluwer, Section I.21.

I. Niven and H. S. Zuckerman, An Introduction to the Theory of Numbers. 2nd ed., Wiley, NY, 1966, p. 94, Problem 11.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

J. J. Sylvester, On the number of fractions contained in any Farey Series of which the Limiting Number is given, London, Edinburgh and Dublin Philosophical Magazine (Fifth Series), vol. 15 (1883), p. 251 = Collected Mathematical Papers, Vols. 1-4, Cambridge Univ. Press, 1904-1912, Vol. 4, p. 103.

A. Walfisz, Weylsche Exponentialsummen in der Neuene Zahlentheorie, VEB Deutsher Verlag, Berlin, 1963

T. D. Noe, Table of n, a(n) for n = 0..1000

S. R. Finch, Euler Totient Function Asymptotic Constants

J. J. Sylvester, The collected mathematical papers of James Joseph Sylvester, vol. 2, vol. 3, vol. 4.

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

Eric Weisstein's World of Mathematics, Beatty Sequence.

Eric Weisstein's World of Mathematics, Totient Summatory Function.

a(n) = (3n^2)/(pi^2) + O( n log n).

More precisely, a(n) = (3/Pi^2)*n^2 + O(n*(log(n))^(2/3)*(log(log(n)))^(4/3)) (A. Walfisz 1963). - Benoit Cloitre (benoit7848c(AT)orange.fr), Feb 02 2003

a(n)=(1/2)*sum(k>=1, mu(k)*floor(n/k)*floor(1+n/k)) - Benoit Cloitre (benoit7848c(AT)orange.fr), Apr 11 2003

The quotient A024916[n]/a[n] = SummatorySigma/SummatoryTotient as n increases seems to approach (Pi^4)/36 = Zeta(2)^2 = 2.705808084277845. See also A067282. - Labos E. (labos(AT)ana.sote.hu), Sep 21 2004

A024916(n)/a(n) = zeta(2)^2 + O(log(n)/n). This follows from asymptotic formulas for the sequences. - Franklin T. Adams-Watters (FrankTAW(AT)Netscape.net), Oct 19 2006

Row sums of triangle A134542 - Gary W. Adamson (qntmpkt(AT)yahoo.com), Oct 31 2007

with(numtheory): A002088 := n->add(phi(i), i=1..n);

Table[Plus @@ EulerPhi[Range[n]], {n, 0, 57}] - Alonso Delarte (alonso.delarte(AT)gmail.com), May 30 2006

Cf. A000010, A015614, A005728, A067282.

Cf. A134542.

Sequence in context: A162578 A152919 A092249 this_sequence A019332 A002491 A045958

Adjacent sequences: A002085 A002086 A002087 this_sequence A002089 A002090 A002091

nonn,easy,nice

N. J. A. Sloane (njas(AT)research.att.com).

Additional comments from Leonard Smiley (smiley(AT)math.uaa.alaska.edu).