A036785: Numbers divisible by the squares of two distinct primes. High-pitched and very harsh. Residue counts: 2 [50439, 6197] 3 [40486, 8099, 8051] 4 [47351, 3131, 3088, 3066] 5 [21657, 8748, 8748, 8734, 8749] 6 [35271, 537, 7606, 5215, 7562, 445] 7 [13842, 7139, 7152, 7121, 7146, 7109, 7127] 8 [23669, 1604, 1567, 1556, 23682, 1527, 1521, 1510] 9 [35112, 2703, 2683, 2704, 2694, 2689, 2670, 2702, 2679] 10 [18029, 652, 8111, 626, 8095, 3628, 8096, 637, 8108, 654] 11 [7639, 4907, 4889, 4894, 4921, 4895, 4894, 4895, 4892, 4912, 4898] 12 [32670, 312, 276, 2633, 7351, 237, 2601, 225, 7330, 2582, 211, 208] Density information: A(x) at x=10,100,1000,...: [0, 3, 49, 554, 5644, 56636] A(x)/x at x=10,100,1000,...: 0.0,0.03,0.049,0.0554,0.05644,0.056636, First 100 first differences: [36, 28, 8, 36, 36, 16, 4, 16, 9, 27, 36, 12, 24, 36, 32, 4, 4, 32, 9, 9, 18, 16, 16, 4, 36, 36, 12, 12, 12, 36, 27, 1, 8, 16, 20, 36, 28, 8, 8, 28, 36, 18, 18, 36, 32, 4, 8, 20, 8, 36, 36, 9, 11, 16, 9, 27, 4, 20, 12, 12, 24, 1, 35, 36, 4, 23, 9, 18, 2, 16, 4, 28, 4, 36, 4, 8, 24, 24, 12, 9, 27, 20, 7, 9, 16, 20, 36, 36, 8, 28, 36, 36, 36, 36, 28, 8, 28, 8, 16, 20] First 99 second differences: [-8,-20,28,0,-20,-12,12,-7,18,9,-24,12,12,-4,-28,0,28,-23,0,9,-2,0,-12,32,0,-24, 0,0,24,-9,-26,7,8,4,16,-8,-20,0,20,8,-18,0,18,-4,-28,4,12,-12,28,0,-27,2,5,-7, 18,-23,16,-8,0,12,-23,34,1,-32,19,-14,9,-16,14,-12,24,-24,32,-32,4,16,0,-12,-3, 18,-7,-13,2,7,4,16,0,-28,20,8,0,0,0,-8,-20,20,-20,8,4] Prominent spectral lines all multiples of 44100/36=1225 hz. This is because the smallest product of two distinct primes is 2^2 * 3^2 which is 36. This means that any number that is a multiple of 36 will be in the sequence. A large number of numbers in the sequence are congruent to 0 mod 2, 3, 4, 6, 9, and 12 because of this. The next smallest number that is a product of the square of two distinct primes is 2^2 * 5^2 which is 100. This results in the significant number of numbers that are congruent to 0 mod 5 and mod 10. --David Oh