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 A225801 #24 Feb 22 2025 23:59:30 %S A225801 51,69,74,77,106,111,115,129,187,226,235,237,267,291,314,355,371,403, %T A225801 407,411,422,427,437,497,519,533,591,597,649,655,669,713,723,763,785, %U A225801 789,831,893,905,939,943,965,1003,1027,1037,1059,1067,1073,1126,1159,1165 %N A225801 The only semiprime s such that 2*a < s < 2*b where a, b are consecutive semiprimes. %H A225801 Alois P. Heinz, <a href="/A225801/b225801.txt">Table of n, a(n) for n = 1..1000</a> %e A225801 74 is the only semiprime in the interval [2*35, 2*38] = [70, 76] where 35, 38 are consecutive semiprimes. %o A225801 (PARI) issemi(n)=bigomega(n)==2 %o A225801 precsemi(n)=while(!issemi(n--),); n %o A225801 nextsemi(n)=while(!issemi(n++),); n %o A225801 is(n)=if(!issemi(n),return(0)); my(a=precsemi(ceil(n/2)),b=nextsemi(n\2)); nextsemi(a)==b && nextsemi(2*a)==n && precsemi(2*b)==n \\ _Charles R Greathouse IV_, Mar 18 2014 %Y A225801 Cf. A001358, A166251, A217561. %K A225801 nonn %O A225801 1,1 %A A225801 _Irina Gerasimova_, Aug 18 2013