A210665 Least semiprime dividing digit reversal of n-th semiprime, or 0 if no such factor.

4, 6, 9, 0, 0, 51, 4, 22, 4, 62, 33, 0, 0, 0, 93, 4, 94, 15, 55, 15, 85, 26, 4, 4, 0, 77, 4, 58, 4, 6, 0, 39, 49, 0, 0, 111, 511, 0, 0, 121, 221, 321, 921, 0, 0, 141, 0, 341, 0, 0, 551, 851, 951, 161, 0, 961, 771, 871, 381, 581, 781, 0, 6, 202, 302, 502, 14

Offset: 1

Author

Jonathan Vos Post, Mar 28 2012

Keywords

• nonn
• base
• look
• easy

Examples

			a(4) = 0 because the 4th semiprime is 10, and R(10) = 1, which is not divisible by any semiprime.
a(6) = 51 because the 6th semiprime is 15, and R(15) = 51, which is itself semiprime.
a(7) = 4 because the 7th semiprime is 21, R(21) = 12, and 4 is the least semiprime divisor of 12.
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001358, A210615, A210616.

Programs

Maple

r:= proc(n) option remember; local q;
      `if`(n<10, n, irem(n, 10, 'q') *10^(length(n)-1)+r(q))
    end:
b:= proc(n) option remember; local k;
      if n=0 then 0
    else for k from b(n-1)+1
           while isprime(k) or 2<>add (i[2], i=ifactors(k)[2])
         do od; k
      fi
    end:
a:= proc(n) option remember; local m, k;
      m:= r(b(n));
      for k from 4 to m do
         if irem(m, k)=0 and not isprime(k) and
            add(i[2], i=ifactors(k)[2])=2 then return k fi
      od; 0
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Mar 28 2012

Formula

a(n) = A210615(A210616(n)).