A239307 Semiprimes n = p*q such that reverse(n)=reverse(p)*reverse(q) where reverse(n) is also semiprime.

4, 6, 9, 22, 26, 33, 39, 55, 62, 77, 93, 121, 143, 169, 187, 202, 226, 262, 303, 339, 341, 393, 505, 622, 626, 707, 781, 933, 939, 961, 1111, 1177, 1243, 1313, 1441, 1469, 1661, 1717, 1991, 2042, 2062, 2066, 2206, 2402, 2426, 2446, 2462, 2602, 2642, 3063, 3093

Offset

1,1

Comments

Subsequence of A001358.

Links

Giovanni Resta, Table of n, a(n) for n = 1..10000

Example

1469 = 13*113 is in the sequence because reverse(1469)=reverse(13)*reverse(113) => 9641 = 31*311 where 31 and 311 are prime numbers.

Maple

with(numtheory):lst:={}:T1:=array(1..300):T2:=array(1..300):k:=0:
   for n from 1 to 1000 do:
       p:=ithprime(n):xp:=convert(p, base, 10):
       np:=nops(xp):sp:=sum('xp[np-i+1]*10^(i-1)', 'i'=1..np):
         if type(sp, prime)=true
         then
         k:=k+1:T1[k]:=p:T2[k]:=sp:
         else
         fi:
    od:
         for i from 1 to k do:
           for j from i to k do:
              x:=T1[i]*T1[j]:y:=convert(x, base, 10):n2:=nops(y):
              s:=sum('y[n2-i+1]*10^(i-1)', 'i'=1..n2):
              if T2[i]*T2[j]=s
              then
              lst:=lst union {x}:
              else
              fi:
          od:
        od:
        print(lst):

Crossrefs

Cf. A007500, A001358.

Sequence in context: A357741 A115681 A257842 * A137253 A035135 A046338

Adjacent sequences: A239304 A239305 A239306 * A239308 A239309 A239310

Keyword

nonn,base

Author

Michel Lagneau, Mar 15 2014

Status

approved