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A337559
Number of length three 1..n vectors that contain their harmonic mean.
2
1, 2, 3, 4, 5, 18, 19, 20, 21, 22, 23, 36, 37, 38, 51, 52, 53, 66, 67, 80, 81, 82, 83, 96, 97, 98, 99, 112, 113, 138, 139, 140, 141, 142, 155, 168, 169, 170, 171, 184, 185, 210, 211, 212, 237, 238, 239, 252, 253, 254, 255, 256, 257, 270, 271, 284, 285, 286, 287, 324, 325, 326, 339
OFFSET
1,2
LINKS
Hywel Normington, Python code, 2020.
FORMULA
Empirical: If A174903(n) = 0, a(n) = a(n-1) + 1.
a(n)-a(n-1) = 1 (mod 6).
EXAMPLE
For n = 1, the only solution is (1,1,1).
For n = 6, the a(6) = 18 solutions are (k,k,k) for k=1,..,6, the 6 permutations of (2,3,6) and the 6 permutations of (3,4,6).
For n = 40, the a(40)-a(39) = 13 new solutions are (40,40,40), the 6 permutations of (10,16,40) and the 6 permutations of (24,30,40).
CROSSREFS
KEYWORD
nonn
AUTHOR
Hywel Normington, Aug 31 2020
STATUS
approved