

A173638


The nth semiprime plus n gives a palindrome in base 10.


1



1, 2, 11, 17, 20, 23, 25, 35, 40, 48, 53, 59, 69, 86, 94, 100, 128, 133, 138, 141, 145, 194, 211, 216, 224, 232, 282, 326, 450, 615, 665, 824, 876, 929, 1171, 1197, 1267, 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
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OFFSET

1,2


COMMENTS

This is to semiprimes A001358 as A115884 is to primes A000040.


LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000


FORMULA

{n: n + A001358(n) is in A002113} == {n: n + A001358(n) = R(n)} == {n: n + A001358(n) = A004086(n)}.


EXAMPLE

a(1) = 1 because 1st semiprime = 4, 4+1=5 is trivially a palindrome.
a(2) = 2 because 2nd semiprime = 6, 6+2=8 is trivially a palindrome.
a(3) = 11 because 11th semiprime = 33, 33+11=44 is nontrivially a palindrome.
a(4) = 17 because 17th semiprime = 49, 49+17=66 is nontrivially a palindrome.
a(5) = 20 because 20th semiprime = 57, 57+20=77 is nontrivially a palindrome.
a(8) = 35 because 35th semiprime = 106, 106+35=141 is nontrivially a palindrome.


MATHEMATICA

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 *)


CROSSREFS

A001358, A002113, A115884, A100493.
Sequence in context: A038927 A105877 A309499 * A018420 A253474 A133410
Adjacent sequences: A173635 A173636 A173637 * A173639 A173640 A173641


KEYWORD

nonn,base


AUTHOR

Jonathan Vos Post, Nov 23 2010


STATUS

approved



