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A171837
Array g(n,k) read by antidiagonals: the k-th integer with prime factorization 2^i * 3^(n-i) * 5^e_5 *7^e_7 * (... higher primes).
0
1, 2, 5, 4, 3, 7, 8, 6, 10, 11, 16, 12, 9, 14, 13, 32, 24, 18, 20, 15, 17, 64, 48, 36, 27, 28, 21, 19, 128, 96, 72, 54, 40, 30, 22, 23, 256, 192, 144, 108, 80, 56, 42, 26, 25, 512, 384, 288, 216, 160, 81, 60, 44, 33, 29, 1024, 768, 576, 432, 320, 162, 112, 84, 45, 34, 31
OFFSET
1,2
EXAMPLE
The array starts in row n=0 as:
1, 5, 7, 11, 13, 17, 19, 23, 25, 29: not divisible by 2 or 3
2, 3, 10, 14, 15, 21, 22, 26, 33, 34: divisible by 2^i*3^(1-i), i<=1
4, 6, 9, 20, 28, 30, 42, 44, 45, 52: divisible by 2^i*3^(2-i), i<=2
8, 12, 18, 27, 40, 56, 60, 84, 88, 90: divisible by 2^i*3^(3-i): i<=3
16, 24, 36, 54, 80, 81, 112, 120, 168, 176
32, 48, 72, 108, 160, 162, 224, 240, 243, 336
64, 96, 144, 216, 320, 324, 448, 480, 486, 672
MATHEMATICA
f[n_] := Plus @@ Last /@ Select[FactorInteger@n, 1 < #[[1]] < 4 &]; g[n_, k_] := Select [Range@ 1100, f@# == n &][[k]]; Table[g[n - k, k], {n, 11}, {k, n}] // Flatten
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Robert G. Wilson v, Dec 19 2009
STATUS
approved