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A239307 Semiprimes n = p*q such that reverse(n)=reverse(p)*reverse(q) where reverse(n) is also semiprime. 1
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 (list; graph; refs; listen; history; text; internal format)
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: A032817 A115681 A257842 * A137253 A035135 A046338

Adjacent sequences:  A239304 A239305 A239306 * A239308 A239309 A239310

KEYWORD

nonn,base

AUTHOR

Michel Lagneau, Mar 15 2014

STATUS

approved

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Last modified August 10 00:32 EDT 2022. Contains 356026 sequences. (Running on oeis4.)