A242592 - OEIS

%I #8 May 19 2014 04:37:39

%S 6,22,26,33,39,55,62,77,93,143,187,202,226,262,303,339,341,393,505,

%T 622,626,707,781,933,939,1111,1177,1243,1313,1441,1469,1661,1717,1991,

%U 2042,2062,2066,2206,2402,2426,2446,2462,2602,2642,3063,3093,3099,3131,3309

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

%C Subsequence of A085751.

%H Michel Lagneau, <a href="/A242592/b242592.txt">Table of n, a(n) for n = 1..999</a>

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

%p for n from 6 to 4000 do :

%p   x:=factorset(n):n1:=nops(x):

%p    if bigomega(n)= 2 and n1>1

%p    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):

%p     if bigomega(s)=2 and nn1>1

%p     then

%p     z:=convert(s,base,10):n3:=nops(z): p1:=x1[1]:q1:=x1[2]:

%p     lst1:={p1} union  {q1}:s1:=sum('z[i]*10^(n3-i)','i'=1..n3):

%p        if lst = lst1

%p         then

%p         printf(`%d, `,n):

%p         else

%p        fi:

%p      fi:

%p     fi:

%p   od:

%Y Cf. A001358, A006881, A085751, A210616.

%K nonn,base

%O 1,1

%A _Michel Lagneau_, May 18 2014