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 A140123 #5 Oct 03 2015 23:27:13
%S A140123 4,12,36,180,1260,252,252,2772,69300,900900,900900,15315300,15315300,
%T A140123 290990700,290990700,6692786100,46849502700,46849502700,46849502700,
%U A140123 46849502700,1358635578300,42117702927300,42117702927300,42117702927300
%N A140123 Denominator of Sum_{k=1..n} (-1)^k / semiprime(k).
%e A140123 The first 10 values of A140122(n)/a(n) = -1/4, -1/12, -7/36, -17/180, -209/1260, -25/252, -37/252, -281/2772, -9797/69300, -92711/900900. The 10th term of the sum is (-1/4)+(1/6)-(1/9)+(1/10)-(1/14)+(1/15)-(1/21)+(1/22)-(1/25)+(1/26) = -92711/900900 hence a(10) = 900900. The 20th term of the alternating sum is (-1/4)+(1/6)-(1/9)+(1/10)-(1/14)+(1/15)-(1/21)+(1/22)-(1/25)+(1/26)-(1/33)+(1/34)-(1/35)+(1/38)-(1/39)+(1/46)-(1/49)+(1/51)-(1/55)+(1/57) = -5218865543/46849502700, hence a(20) = 46849502700.
%p A140123 A001358 := proc(n) local a; if n = 1 then 4; else for a from A001358(n-1)+1 do if numtheory[bigomega](a) = 2 then RETURN(a) ; fi ; od: fi ; end: A140123 := proc(n) local k ; denom(add ( (-1)^k/A001358(k),k=1..n)) ; end: seq(A140123(n),n=1..30) ; # _R. J. Mathar_, May 13 2008
%Y A140123 Cf. A001358, A002110, A024530, A140122.
%K A140123 easy,frac,nonn
%O A140123 1,1
%A A140123 _Jonathan Vos Post_, May 09 2008
%E A140123 More terms from _R. J. Mathar_, May 13 2008