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 A045973 #14 Sep 19 2023 01:43:44
%S A045973 10,21,55,441,91,1155,187,9261,3025,1911,247,24255,391,3927,5005,
%T A045973 194481,551,63525,713,40131,10285,5187,1073,509355,8281,8211,166375,
%U A045973 82467,1271,105105,1591,4084101,13585,11571,17017,1334025,1927,14973,21505,842751
%N A045973 a(1)=10; if n = Product p_i^e_i, n > 1, then a(n) = Product p_{i+1}^e_i * Product p_{i+3}^e_i.
%D A045973 From a puzzle proposed by _Marc LeBrun_.
%F A045973 Sum_{n>=1} 1/a(n) = -9/10 + Product_{k>=1} (1+1/(prime(k)*prime(k+4)-1)) = 0.2602421684... . - _Amiram Eldar_, Sep 19 2023
%t A045973 f[p_, e_] := NextPrime[p]^e * NextPrime[p, 3]^e; a[1] = 10; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* _Amiram Eldar_, Sep 19 2023 *)
%Y A045973 Cf. A045965, A045966, A045967, A045968, A045969, A045970, A045971, A045972.
%K A045973 easy,nonn
%O A045973 1,1
%A A045973 _N. J. A. Sloane_
%E A045973 More terms from _David W. Wilson_