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 A108728 #52 Apr 21 2024 10:01:53 %S A108728 1,2,3,2,5,3,2,3,6,5,3,4,6,4,5,4,3,5,4,7,6,5,8,3,7,4,4,8,5,7,4,4,8,5, %T A108728 7,6,4,8,8,7,5,3,7,4,6,10,11,5,4,10,6,5,6,5,5,4,9,4,8,9,6,5,8,4,12,5, %U A108728 4,8,10,5,9,7,6,8,8,5,10,5,9,7,5,5,8,3,11,6,6,7,9,10 %N A108728 Number of distinct prime divisors of concatenated even numbers. %H A108728 Apurva Rai and Michael S. Branicky, <a href="/A108728/a108728.txt">Prime Factorizations of OEIS A019520</a>. %F A108728 a(n) = A001221(A019520(n)). - _Amiram Eldar_, Jan 27 2020 %e A108728 2 has 1 distinct prime divisors, so a(1) = 1. %e A108728 24 has 2 distinct prime divisors, so a(2) = 2. %e A108728 246 has 3 distinct prime divisors, so a(3) = 3. %t A108728 a[n_] := PrimeNu @ FromDigits @ Flatten[IntegerDigits /@ (2*Range[n])]; Array[a, 30] (* _Amiram Eldar_, Jan 27 2020 *) %Y A108728 Cf. A001221, A019520, A105388 (number of divisors). %K A108728 nonn,base %O A108728 1,2 %A A108728 _Parthasarathy Nambi_, Jun 21 2005 %E A108728 a(43)-a(52) from _Amiram Eldar_, Jan 27 2020 %E A108728 a(53)-a(54) from _Jinyuan Wang_, Jun 27 2020 %E A108728 a(55)-a(69) from _Apurva Rai_ and _Michael S. Branicky_, Aug 16 2020 %E A108728 a(70)-a(71) from _Max Alekseyev_, Mar 21 2023 %E A108728 a(72)-a(89) from _Tyler Busby_, Mar 23 2023 %E A108728 a(90) from _Tyler Busby_, Apr 20 2024