Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #27 Mar 14 2024 15:21:40
%S 9,4,6,10,69,15,26,169,146,237,95,1082,818,597,1603,2705,2078,4511,
%T 1418,2681,14545,13863,37551,6559,16053,55805,26707,17965,308918,
%U 32777,41222,35103,393565,219509,153263,87627,2263057,35981,1789339,741841,797542
%N a(n) is the smallest semiprime such that difference between a(n) and next semiprime, b(n), is n.
%C This is the semiprime analogous to A000230. - _Robert G. Wilson v_, Jun 13 2013
%H Martin Raab, <a href="/A131109/b131109.txt">Table of n, a(n) for n = 1..120</a>, terms up to a(100) from T. D. Noe and Klaus Brockhaus
%F a(n) = A001358(A123375(n)). - _T. D. Noe_, Sep 28 2007
%e n, b(n)-a(n): 1=10-9, 2=6-4, 3=9-6, 4=14-10, 5=74-69, 6=21-15, 7=33-26, 8=177-169, 9=155-146, 10=247-237, 11=106-95, 12=1094-1082, 13=831-818, 14=611-597, 15=1618-1603, 16=2721-2705, 17=2095-2078, 18=4529-4511, 19=1437-1418, 20=2701-2681, 21=14566-14545, 22=13885-13863, 23=37574-37551, 24=6583-6559, 25=16078-16053, 26=55831-55805, 27=26734-26707, 28=17993-17965, 29=308947-308918, 30=32807-32777, 31=41253-41222, 32=35135-35103, 33=393598-393565, 34=219543-219509, 35=153298-153263, 36=87663-87627, 37=2263094-2263057, 38=36019-35981.
%t SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; NextSemiPrime[n_] := Module[{m = n + 1}, While[! SemiPrimeQ[m], m++]; m]; nn = 30; t = Table[0, {nn}]; found = 0; sp0 = 4; While[found < nn, sp1 = NextSemiPrime[sp0]; d = sp1 - sp0; If[d <= nn && t[[d]] == 0, t[[d]] = sp0; found++]; sp0 = sp1]; t (* _T. D. Noe_, Oct 02 2012 *)
%Y Cf. A065516, A133478.
%K nonn
%O 1,1
%A _Zak Seidov_, Sep 24 2007
%E Corrected and extended by _T. D. Noe_ and _R. J. Mathar_, Sep 28 2007