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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
OFFSET
1,1
COMMENTS
Subsequence of A001358.
LINKS
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
Sequence in context: A357741 A115681 A257842 * A137253 A035135 A046338
KEYWORD
nonn,base
AUTHOR
Michel Lagneau, Mar 15 2014
STATUS
approved