%I #12 Aug 11 2024 14:41:34
%S 1212121,12121212121,1212121212121212121212121212121212121212121
%N Smoothly undulating palindromic primes of the form (12*10^n-21)/99.
%C The De Geest link calls these smoothly undulating palindromic primes. Corresponding n are given in A062209. Equivalently, primes of the form 1212...121: Decimal digits "12" repeated k times with 1 appended (or "21" repeated k times with 1 prefixed). Corresponding k are given in A056803. The next term, a(4), has "12" repeating A056803(4) = 69 times and length A062209(4) = 2*A056803(4) + 1 = 139 decimal digits.
%H P. De Geest, <a href="https://www.worldofnumbers.com/undulat.htm">SUPP Reference Table</a>
%H <a href="/index/Pri#pri_supp">Index entries for sequences related to smoothly undulating palindromic primes</a>
%F a(n) = (4*10^A062209(n)-7)/33. - _M. F. Hasler_, Jul 30 2015
%Y Cf. A056803 (number of 12's (or 21's)), A062209 (corresponding decimal digit lengths).
%Y Cf. A032758, A033619, A030291, A059168, A242541, A004022.
%K bref,nonn,base
%O 1,1
%A _Rick L. Shepherd_, Mar 04 2004
%E Edited by _M. F. Hasler_, Jul 30 2015