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A368249
a(n) = A002378(A005117(n)-1).
4
0, 2, 6, 20, 30, 42, 90, 110, 156, 182, 210, 272, 342, 420, 462, 506, 650, 812, 870, 930, 1056, 1122, 1190, 1332, 1406, 1482, 1640, 1722, 1806, 2070, 2162, 2550, 2756, 2970, 3192, 3306, 3422, 3660, 3782, 4160, 4290, 4422, 4692, 4830, 4970, 5256, 5402, 5852, 6006
OFFSET
1,2
COMMENTS
The squarefree oblong numbers (A229882) are all terms of this sequence, and their relative asymptotic density in it is A065474/A059956 = 0.530711... (A065469).
LINKS
FORMULA
Sum_{n>=2} 1/a(n) = Sum_{k>=2} (zeta(k)/zeta(2*k) - 1) = 0.848633... (A368250).
MATHEMATICA
Table[n*(n - 1), {n, Select[Range[100], SquareFreeQ]}]
PROG
(PARI) lista(kmax) = forsquarefree(k=1, kmax, print1(k[1]*(k[1]-1), ", "));
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Amiram Eldar, Dec 19 2023
STATUS
approved