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).

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

Links

Table of n, a(n) for n=1..66.

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

Cf. A169611, A171126, A171127.

Sequence in context: A265358 A361832 A361945 * A216253 A115303 A266403

Adjacent sequences: A171834 A171835 A171836 * A171838 A171839 A171840

Keyword

nonn,tabl

Author

Robert G. Wilson v, Dec 19 2009

Status

approved