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 A128281 #18 Aug 29 2020 02:48:24 %S A128281 3,21,105,1365,884037 %N A128281 a(n) is the least product of n distinct odd primes m=p_1*p_2*...*p_n, such that (d+m/d)/2 are all primes for each d dividing m. %C A128281 From _Iain Fox_, Aug 26 2020: (Start) %C A128281 a(6) > 10^9 if it exists. %C A128281 All terms are members of A076274 since the definition requires that (1+m)/2 be prime. %C A128281 The number of prime factors of m congruent to 3 (mod 4) must be even except for n=1. %C A128281 (End) %C A128281 a(6) > 2*10^11 if it exists. - _David A. Corneth_, Aug 27 2020 %C A128281 a(n) >= A070826(n+1) by definition of the sequence. - _Iain Fox_, Aug 28 2020 %e A128281 105=3*5*7, (3*5*7+1)/2=53, (3+5*7)/2=19, (5+3*7)/2=13, (7+3*5)/2=11 are all primes and 105 is the least such number which is the product of 3 primes, so a(3)=3. %o A128281 (PARI) a(n)=if(n==1, return(3)); my(p=prod(k=1, n, prime(k+1))); forstep(m=p+if(p%4-1, 2), +oo, 4, if(bigomega(m)==n && omega(m)==n, fordiv(m, d, if(!isprime((d+m/d)/2), next(2))); return(m))) \\ _Iain Fox_, Aug 27 2020 %Y A128281 Subsequence of A076274. %Y A128281 Lower bound: A070826. %Y A128281 Cf. A002145, A128276, A128282, A128283, A128284, A128285, A128286, A168352. %K A128281 nonn,hard,more %O A128281 1,1 %A A128281 Kok Seng Chua (chuakokseng(AT)hotmail.com), Mar 05 2007 %E A128281 Definition corrected by _Iain Fox_, Aug 25 2020