%N Let f denote the map that replaces k with the concatenation of its proper divisors, written in increasing order, each divisor being written in base 10 in the normal way. Then a(n) = prime reached when starting at 2n+1 and iterating f.
%C If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1.
%e n = 7: 2n+1 = 15 = 3*5 -> 35 = 5*7 -> 57 = 3*19 -> 319 = 11*29 -> 1129, prime, so a(7) = 1129.
%Y Cf. A130140, A130141, A130142. A bisection of A120716.
%A _N. J. A. Sloane_, Jul 30 2007
%E The value of a(22) is currently unknown.