A245044 - OEIS

%I #15 Jul 16 2014 10:59:36

%S 4,6,9,86,753,7531,9753,97531

%N Semiprimes with digits in descending order that differ exactly by 2.

%e 7531 is in the sequence because it is semiprime 7531 = 17*443 and 7-5 = 5-3 = 3-1 = 2.

%e 97531 is in the sequence because it is semiprime 97531 = 7*13933 and 9-7 = 7-5 = 5-3 = 3-1 = 2.

%t Sort[Flatten[Table[Select[FromDigits/@Partition[Range[9, 0, -2], n, 1], PrimeOmega[#]==2 &], {n, 1000}]]]

%o (PARI) for(d=1,5,for(s=1,11-2*d,t=sum(i=1,d,(s-2+2*i)*10^i)/10; if(bigomega(t)==2, print1(t", ")))) \\ _Charles R Greathouse IV_, Jul 16 2014

%Y Cf. A215477, A215478.

%K nonn,base,easy,fini,full

%O 1,1

%A _Vincenzo Librandi_, Jul 10 2014

%E a(1), a(2), a(4) from _Charles R Greathouse IV_, Jul 16 2014