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 A291356 #17 Dec 30 2020 12:02:40 %S A291356 1,0,5,18,164,1538,20018,408404,7759748,148728578,0,194482899488,0, %T A291356 286353189202064,0,491911826070793124,32087786808310197824, %U A291356 1809656800145141307344,113734187984879487014048,0,0,27153120399499349433747548978,0 %N A291356 a(n) is the smallest k such that usigma(k) = A002110(n), or 0 if no such k exists. %e A291356 a(3) = 18 because usigma(18) = 30 = A002110(3) and 18 is the smallest number with this property. %o A291356 (PARI) nthprimorial(n)=prod(i=1,n, prime(i)); %o A291356 usigma(n,f=factor(n))=prod(i=1,#f~, f[i,1]^f[i,2]+1); %o A291356 a(n)=my(target=nthprimorial(n)); forfactored(n=target*nthprimorial(n-1)\usigma(nthprimorial(n-1)),target, if(usigma(n[1],n[2])==target, return(n[1]))); 0 \\ _Charles R Greathouse IV_, Aug 23 2017 %Y A291356 Cf. A002110, A034448, A289126. %K A291356 nonn,more %O A291356 0,3 %A A291356 _Amiram Eldar_ and _Altug Alkan_, Aug 23 2017 %E A291356 a(10)-a(18) from _Giovanni Resta_, Aug 23 2017 %E A291356 a(19)-a(22) from _Daniel Suteu_, Dec 29 2020