A242592 Squarefree semiprimes, n=p*q, where reversal(n) is semiprime and reversal(n) = reversal(p)*reversal(q).

6, 22, 26, 33, 39, 55, 62, 77, 93, 143, 187, 202, 226, 262, 303, 339, 341, 393, 505, 622, 626, 707, 781, 933, 939, 1111, 1177, 1243, 1313, 1441, 1469, 1661, 1717, 1991, 2042, 2062, 2066, 2206, 2402, 2426, 2446, 2462, 2602, 2642, 3063, 3093, 3099, 3131, 3309

Offset: 1

Author

Michel Lagneau, May 18 2014

Keywords

• nonn
• base

Comments

Subsequence of A085751.

Examples

			1469 = 13*113 is in the sequence because reversal(1469) = 9641 = 31*311 where 31 = reversal(13) and 311 = reversal(113).
		

Links

• Michel Lagneau, Table of n, a(n) for n = 1..999

Crossrefs

Cf. A001358, A006881, A085751, A210616.

Programs

Maple

for n from 6 to 4000 do :
  x:=factorset(n):n1:=nops(x):
   if bigomega(n)= 2 and n1>1
   then                 y:=convert(n,base,10):n2:=nops(y):p:=x[1]:q:=x[2]:xp1:=convert(p,base,10):nxp1:=nops(xp1):xq1:=convert(q,base,10):nxq1:=nops(xq1):sp:=sum('xp1[i]*10^(nxp1-i)', 'i'=1..nxp1):sq:=sum('xq1[i]*10^(nxq1-i)', 'i'=1..nxq1):lst:={sp} union {sq}:s:=sum('y[i]*10^(n2-i)', 'i'=1..n2):x1:=factorset(s):nn1:=nops(x1):
    if bigomega(s)=2 and nn1>1
    then
    z:=convert(s,base,10):n3:=nops(z): p1:=x1[1]:q1:=x1[2]:
    lst1:={p1} union  {q1}:s1:=sum('z[i]*10^(n3-i)','i'=1..n3):
       if lst = lst1
        then
        printf(`%d, `,n):
        else
       fi:
     fi:
    fi:
  od: