Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #11 Oct 08 2016 11:34:47
%S 1,2,11,17,20,23,25,35,40,48,53,59,69,86,94,100,128,133,138,141,145,
%T 194,211,216,224,232,282,326,450,615,665,824,876,929,1171,1197,1267,
%U 1290,1293,1450,1498,1520,1566,1655,1790,1898,2248,2313,2624,2786,2826,2849,2912,3058,3082,3098,3270,3290,3408,3586,3610,3672,3792,3912,3945,3982,4000
%N The n-th semiprime plus n gives a palindrome in base 10.
%C This is to semiprimes A001358 as A115884 is to primes A000040.
%H Harvey P. Dale, <a href="/A173638/b173638.txt">Table of n, a(n) for n = 1..1000</a>
%F {n: n + A001358(n) is in A002113} == {n: n + A001358(n) = R(n)} == {n: n + A001358(n) = A004086(n)}.
%e a(1) = 1 because 1st semiprime = 4, 4+1=5 is trivially a palindrome.
%e a(2) = 2 because 2nd semiprime = 6, 6+2=8 is trivially a palindrome.
%e a(3) = 11 because 11th semiprime = 33, 33+11=44 is nontrivially a palindrome.
%e a(4) = 17 because 17th semiprime = 49, 49+17=66 is nontrivially a palindrome.
%e a(5) = 20 because 20th semiprime = 57, 57+20=77 is nontrivially a palindrome.
%e a(8) = 35 because 35th semiprime = 106, 106+35=141 is nontrivially a palindrome.
%t Module[{nn=20000,sems},sems=Select[Range[nn],PrimeOmega[#]==2&]; Select[ Thread[{Range[Length[sems]],sems}],Total[ #]==IntegerReverse[Total[ #]]&]] [[All,1]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Oct 08 2016 *)
%Y A001358, A002113, A115884, A100493.
%K nonn,base
%O 1,2
%A _Jonathan Vos Post_, Nov 23 2010