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

1,1

Links

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

Formula

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

Example

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.

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

Crossrefs

Cf. A001358, A210615, A210616.

Sequence in context: A095211 A141569 A200621 * A228586 A200107 A198733

Adjacent sequences: A210662 A210663 A210664 * A210666 A210667 A210668

Keyword

nonn,base,look,easy

Author

Jonathan Vos Post, Mar 28 2012

Status

approved