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 A239038 #28 Mar 14 2014 11:33:22 %S A239038 9,14,49,55,94,115,446,611,869,961,4031,4315,7891,7934,8143,11651, %T A239038 16129,16255,32254,37301,51089,54701,60311,64931,65279,65441,241519, %U A239038 287509,321029,367459,384799,446201,495409,513847,521029,808691,1297915,1582619,1685219,1883681 %N A239038 Semiprimes of the form (2^k - m)*(m*2^k - 1). %H A239038 Charles R Greathouse IV, <a href="/A239038/b239038.txt">Table of n, a(n) for n = 1..10000</a> %e A239038 9 is in this sequence because (2^1-1)*(1*2^1-1) = 3*3 = 9 is semiprime for k=1 and m=1, %e A239038 49 is in this sequence because (2^3-1)*(1*2^3-1) = 7*7 = 49 is semiprime for k=3 and m=1, %e A239038 115 is in this sequence because (2^3-3)*(3*2^3-1) = 5*23 = 115 is semiprime for k=3 and m=3. %o A239038 (PARI) list(lim)=my(v=List(),t); for(k=1,log(sqrt(lim)+2)\log(2), for(m=1, min((lim+1)>>k,2^k-2),my(a=2^k-m,b=m<<k-1,n=a*b); if(n<=lim && isprime(a) && isprime(b), listput(v,n))); t=4^k-2^k-1; if(t<=lim && bigomega(t)==2,listput(v, t))); Set(v) \\ _Charles R Greathouse IV_, Mar 11 2014 %Y A239038 Cf. A000668 (Mersenne primes). %K A239038 nonn %O A239038 1,1 %A A239038 _Juri-Stepan Gerasimov_, Mar 09 2014 %E A239038 Missing terms inserted by _Charles R Greathouse IV_, Mar 11 2014