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 A309396 #22 Nov 25 2021 01:12:05 %S A309396 1,1,1,2,7,26,115,614,3866,28339,237017,2227657,23233568,266201749 %N A309396 Number of lucky numbers <= n!. %e A309396 a(1) = 1 because there is one lucky number (1) <= 1 (1!). %e A309396 a(3) = 2 because there are two lucky numbers (1, 3) <= 6 (3!). %o A309396 (SageMath) %o A309396 def lucky(n): %o A309396 L=list(range(1, n+1, 2)); j=1 %o A309396 while L[j] <= len(L)-1: %o A309396 L=[L[i] for i in range(len(L)) if (i+1)%L[j]!=0] %o A309396 j+=1 %o A309396 return(L) %o A309396 A000959=lucky(factorial(10)) %o A309396 def lucky_range(a,b): %o A309396 lucky = [] %o A309396 for l in A000959: %o A309396 if l >= b: %o A309396 return lucky %o A309396 if l>=a: lucky.append(l) %o A309396 [ len(lucky_range(0,factorial(n)+1)) for n in range(10) ] %Y A309396 Cf. A000959, A003604, A309399, A000142. %K A309396 nonn,hard,more %O A309396 0,4 %A A309396 _Hauke Löffler_, Jul 28 2019 %E A309396 a(10)-a(12) from _Giovanni Resta_, May 10 2020 %E A309396 a(13) from _Kevin P. Thompson_, Nov 24 2021