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A018806 Sum of gcd(x, y) for 1 <= x, y <= n. 5
1, 5, 12, 24, 37, 61, 80, 112, 145, 189, 220, 288, 325, 389, 464, 544, 593, 701, 756, 880, 989, 1093, 1160, 1336, 1441, 1565, 1700, 1880, 1965, 2205, 2296, 2488, 2665, 2829, 3028, 3328, 3437, 3621, 3832, 4152, 4273, 4621, 4748, 5040, 5373, 5597, 5736, 6168 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) is also the entrywise 1-norm of the n X n GCD matrix.

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

Sum_{k=1..n} phi(k)*(floor(n/k))^2. - Vladeta Jovovic, Nov 10 2002

a(n) ~ kn^2 log n, with k = 6/Pi^2. - Charles R Greathouse IV, Jun 21 2013

G.f.: sum(k >= 1, phi(k)*x^k*(1+x^k)/((1-x^k)^2*(1-x)). - Robert Israel, Jan 14 2015

MAPLE

N:= 1000 # to get a(1) to a(N)

g:= add(numtheory:-phi(k)*x^k*(1+x^k)/((1-x^k)^2*(1-x)), k=1..N):

S:= series(g, x, N+1):

seq(coeff(S, x, j), j=1..N); # Robert Israel, Jan 14 2015

MATHEMATICA

Table[nn = n; Total[Level[Table[Table[GCD[i, j], {i, 1, nn}], {j, 1, nn}], {2}]], {n, 1, 48}] (* Geoffrey Critzer, Jan 14 2015 *)

PROG

(PARI) a(n)=2*sum(i=1, n, sum(j=1, i-1, gcd(i, j)))+n*(n+1)/2 \\ Charles R Greathouse IV, Jun 21 2013

(PARI) a(n)=sum(k=1, n, eulerphi(k)*(n\k)^2) \\ Charles R Greathouse IV, Jun 21 2013

CROSSREFS

Cf. A000010, A018805, A064951.

Sequence in context: A270681 A212540 A100479 * A101425 A191831 A188182

Adjacent sequences:  A018803 A018804 A018805 * A018807 A018808 A018809

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified December 15 00:30 EST 2019. Contains 329988 sequences. (Running on oeis4.)