A393301 - OEIS

%I #7 Feb 17 2026 21:24:30

%S 0,5,18,87,311,11249

%N a(1) = 0; for n > 1, a(n) is the smallest unused number k such that k! contains the concatenation of a(1), a(2), ..., a(n-1) as a substring.

%F a(n) > a(n-1).

%e a(4) = 87 as 87! = 2107757...000000 (133 digits) which contains a(1)|a(2)|a(3) = '0518' as a substring.

%Y Cf. A000142, A393300, A385673, A346120, A392010, A392011.

%K nonn,base,hard,more

%O 1,2

%A _Scott R. Shannon_, Feb 10 2026