A182180 Semiprimes that become prime when their digits are sorted into nonincreasing order.

14, 34, 35, 38, 118, 119, 121, 133, 134, 142, 143, 145, 146, 166, 194, 214, 215, 218, 314, 334, 341, 346, 358, 361, 365, 377, 386, 395, 398, 413, 415, 437, 451, 473, 514, 517, 538, 583, 614, 634, 635, 671, 734, 737, 778, 779, 791, 799, 818, 835, 838, 878, 893

Offset: 1

Author

Jonathan Vos Post, Apr 23 2012

Keywords

• nonn
• base
• easy

Comments

Suggested by Kevin L. Schwartz.

Examples

			a(10) = 121 = 11*11, which becomes the prime 211 when its digits are sorted into nonincreasing order.
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000040, A001358, A115670 Semiprimes (A001358) whose digit reversal is prime, A182150 Semiprimes that are also semiprime when their digits are sorted into nondecreasing order.

Programs

Maple

h:= proc(m) local k; for k from m+1 while isprime(k) or
            add(i[2], i=ifactors(k)[2])<>2 do od; k
    end:
a:= proc(n) option remember; local k;
      k:= h(a(n-1));
      do if isprime(parse(cat(sort(convert(k, base, 10), `>`)[])))
            then return k else k:=h(k) fi
      od
    end: a(0):=0:
seq(a(n), n=1..80);  # Alois P. Heinz, Apr 23 2012

Extensions

More terms from Alois P. Heinz, Apr 23 2012