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 A072665 #17 Jun 23 2017 01:01:42
%S A072665 2,11,2917
%N A072665 Center of smallest run of 2n+1 consecutive numbers with exactly n+1,n,...,2,1,2,...,n,n+1 distinct prime factors, respectively.
%C A072665 Borrowing from musical terminology, these could be considered "swells" of primality - first a crescendo ("more prime"), then a decrescendo ("less prime"). a(3), if it exists, is greater than 70750000. The corresponding sequence but counting prime factors with multiplicity (A001222) has only two terms (2, 5) because either the number immediately before or after any odd center > 5 equals 4k for some k >= 2, and thus has at least three prime factors, not exactly two, when duplicates are counted.
%C A072665 a(3) > 10^63. - _Hiroaki Yamanouchi_, Sep 25 2014
%e A072665 a(0) = 2 (prime) is the smallest number with one prime factor. a(1) = 11 as 10 (=2*5), 11 (prime) and 12 (=2^2*3) have 2,1,2 distinct prime factors (A001221), respectively and there is no smaller center of such a run. a(2) = 2917 as 2915 (=5*11*53), 2916 (=2^2*3^6), 2917 (prime), 2918 (=2*1459) and 2919 (=3*7*139) have 3,2,1,2,3 distinct prime factors and there is no smaller such run.
%Y A072665 Cf. A072664 (smallest finish with run pattern n, ..., 2, 1), A086560 (smallest start with run pattern 1, 2, ..., n), A001221 (omega).
%K A072665 hard,nonn,more,bref
%O A072665 0,1
%A A072665 _Rick L. Shepherd_, Jul 30 2002
%E A072665 Comment expanded and small typos fixed by _Rick L. Shepherd_, Jun 22 2017