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 A049691 a(n)=T(n,n), array T as in A049687. Also a(n)=T(2n,2n), array T given by A049639. 22
 0, 3, 5, 9, 13, 21, 25, 37, 45, 57, 65, 85, 93, 117, 129, 145, 161, 193, 205, 241, 257, 281, 301, 345, 361, 401, 425, 461, 485, 541, 557, 617, 649, 689, 721, 769, 793, 865, 901, 949, 981, 1061, 1085, 1169, 1209, 1257, 1301, 1393, 1425, 1509, 1549 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) is related to the sequence b(n) = |{(x, y): gcd(x, y) = 1, 1<=x, y<=n}| (A018805) as follows: a(n) = b(n - 1) + 2 (for n > 1). - Shawn Westmoreland (westmore(AT)math.utexas.edu), Jun 11 2003 Comment from N. J. A. Sloane, Sep 08 2019 (Start) The above comment can be rephrased as saying that a(n) is the cardinality of the subsequence F(B(2n),n) of the Farey series F(2n) that is extensively studied in Matseev (2017). See the definition on page 1. For example, F(B(2),1), F(B(4),2), F(B(6),3), and F(B(8),4) are: [0, 1/2, 1], [0, 1/3, 1/2, 2/3, 1], [0, 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 1], [0, 1/5, 1/4, 1/3, 2/5, 3/7, 1/2, 4/7, 3/5, 2/3, 3/4, 4/5, 1], of cardinalities 3,5,9,13 respectively. See also A324796/A324797.(End) REFERENCES A. O. Matveev, Farey Fractions, De Gruyter, 2017. LINKS Andrew Howroyd, Table of n, a(n) for n = 0..1000 FORMULA a(n) = A206297(n+1) = 2 + A018805(n) for n > 0. - Andrew Howroyd, Sep 17 2017 MAPLE Farey := proc(n) sort(convert(`union`({0}, {seq(seq(m/k, m=1..k), k=1..n)}), list)) end: # A006842/A006843 BF := proc(m) local a, i, h, k; global Farey; a:=[]; for i in Farey(2*m) do h:=numer(i); k:=denom(i); if (h <= m) and (k-m <= h) then a:=[op(a), i]; fi; od: a; end; [seq(nops(BF(m), m=1..20)]; # this sequence - N. J. A. Sloane, Sep 08 2019 MATHEMATICA a = 0; a[n_] := 2 + Sum[Quotient[n, g]^2*MoebiusMu[g], {g, 1, n}]; Table[a[n], {n, 0, 50}] (* Jean-François Alcover, Oct 07 2017, translated from PARI *) PROG (PARI) a(n) = if(n>0, 2, 0) + sum(g=1, n, (n\g)^2 * moebius(g)); \\ Andrew Howroyd, Sep 17 2017 CROSSREFS Cf. A018805, A049687, A324796, A324797. A206297 is an essentially identical sequence. Sequence in context: A249424 A076274 A058989 * A206297 A320596 A227565 Adjacent sequences:  A049688 A049689 A049690 * A049692 A049693 A049694 KEYWORD nonn AUTHOR EXTENSIONS Terms a(41) and beyond from Andrew Howroyd, Sep 17 2017 STATUS approved

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Last modified November 12 14:35 EST 2019. Contains 329058 sequences. (Running on oeis4.)