This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A067870 #26 Feb 27 2023 04:00:28
%S A067870 7,13,27,33,47,53,67,73,87,93,107,113,127,133,147,153,167,173,187,193,
%T A067870 207,213,227,233,247,253,267,273,287,293,307,313,327,333,347,353,367,
%U A067870 373,387,393,407,413,427,433,447,453,467,473,487,493,507,513,527,533
%N A067870 Numbers k such that k and 3^k end with the same digit.
%C A067870 Also numbers k such that k^k ends with 3. - _Bruno Berselli_, Dec 11 2018
%C A067870 Numbers congruent to {7, 13} mod 20. - _Amiram Eldar_, Feb 27 2023
%H A067870 Colin Barker, <a href="/A067870/b067870.txt">Table of n, a(n) for n = 1..1000</a>
%H A067870 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A067870 a(2*n+1) = 20*n-13, a(2*n) = 20*n-7.
%F A067870 a(n) = 20*(n-1)-a(n-1) for n>1, a(1)=7. - _Vincenzo Librandi_, Aug 08 2010
%F A067870 From _Colin Barker_, Apr 06 2020: (Start)
%F A067870 G.f.: x*(7 + 6*x + 7*x^2) / ((1 - x)^2*(1 + x)).
%F A067870 a(n) = -5 - 2*(-1)^n + 10*n for n>0.
%F A067870 a(n) = a(n-1) + a(n-2) - a(n-3) for n>3. (End)
%F A067870 Sum_{n>=1} (-1)^(n+1)/a(n) = tan(3*Pi/20)*Pi/20. - _Amiram Eldar_, Feb 27 2023
%e A067870 3^13 = 1594323 hence 13 is in the sequence.
%t A067870 LinearRecurrence[{1, 1, -1}, {7, 13, 27}, 50] (* _Amiram Eldar_, Feb 27 2023 *)
%o A067870 (PARI) a(n) = (5*(2*n-1)*(-1)^n - 2)*(-1)^n; \\ _Jinyuan Wang_, Apr 06 2020
%o A067870 (PARI) Vec(x*(7 + 6*x + 7*x^2) / ((1 - x)^2*(1 + x)) + O(x^50)) \\ _Colin Barker_, Apr 06 2020
%K A067870 nonn,base,easy
%O A067870 1,1
%A A067870 _Benoit Cloitre_, Mar 07 2002