This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A219556 #17 Jul 13 2023 01:52:41
%S A219556 4,9,10,15,17,25,26,28,33,35,37,49,55,65,82,95,97,129,143,145,161,163,
%T A219556 215,217,287,289,323,325,485,487,511,513,649,767,769,865,973,1457,
%U A219556 1459,1535,1537,1727,1729,1943,1945,2047,2049,2186,2188,2305,3071,3073,3455,3457
%N A219556 Semiprimes neighboring a 3-smooth number.
%C A219556 This is to A219528 as semiprime A001358 are to primes A000040.
%C A219556 Semiprime numbers of the form of 2^j*3^k +/- 1.
%H A219556 Amiram Eldar, <a href="/A219556/b219556.txt">Table of n, a(n) for n = 1..4377</a> (terms below 10^60)
%e A219556 a(1) = (2^0)*(3^1) + 1 = (2^2)*(3^0) - 1 = 4 = 2*2, a semiprime.
%e A219556 a(2) = (2^3)*(3^0) + 1 = 9 = 3*3.
%e A219556 a(3) = (2^0)*(3^2) + 1 = 10 = 2*5.
%e A219556 a(4) = (2^4)*(3^0) - 1 = 15 = 3*5.
%t A219556 mx = 4000; A003586 = Flatten@ Table[2^i*3^j, {i, 0, Log[2, mx]}, {j, 0, Log[3, mx/2^i]}]; Union@ Join[ Select[A003586, PrimeOmega[# - 1] == 2 &] - 1, Select[A003586, PrimeOmega[# - 1] == 2 || PrimeOmega[# + 1] == 2 &] + 1] (* _Robert G. Wilson v_, Nov 22 2012 *)
%Y A219556 Cf. A001358, A003586, A219528.
%K A219556 nonn,easy
%O A219556 1,1
%A A219556 _Jonathan Vos Post_, Nov 22 2012