%N a(1) = 1; a(n) = smallest positive integer not already used which has a prime signature different from both a(n-1) and a(n-1)+1.
%e To compute a(2), we see a(1) is 1; we look for the smallest unused positive integer that does not have the same prime signature as either 1 or 2. The first such number is 4.
%Y Cf. A179372
%A _Will Nicholes_, Jul 11 2010