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Semiprimes whose prime factors are the digit reversal of each other.
1

%I #8 Oct 04 2024 00:23:42

%S 4,9,25,49,121,403,1207,2701,7663,10201,17161,22801,32761,35143,36481,

%T 75007,97969,117907,124609,127087,139129,140209,146689,173809,197209,

%U 247021,257821,342127,382387,528529,573049,619369,635209,643063,692443,743623,844561,863041

%N Semiprimes whose prime factors are the digit reversal of each other.

%C The squares of all palindromic primes (A002385) are a subsequence and these are the only perfect squares.

%H Andrew Howroyd, <a href="/A376746/b376746.txt">Table of n, a(n) for n = 1..10000</a>

%e 121 is a term because 121 = 11 * 11.

%e 403 is a term because 403 = 13 * 31.

%e 1207 is a term because 1207 = 17 * 71.

%e 2701 is a term because 2701 = 37 * 73.

%o (PARI) upto(lim)={my(L=List()); forprime(p=2, sqrtint(lim), my(q=fromdigits(Vecrev(digits(p)))); if(isprime(q) && p*q<=lim, listput(L,p*q))); Set(L)}

%Y A083815 is a subsequence.

%Y Cf. A002385, A004086.

%K nonn,base

%O 1,1

%A _Andrew Howroyd_, Oct 03 2024