%I #15 Oct 12 2023 18:51:26
%S 7,23,131,47,643,2459,2000807,503347241,82125909539251,9617692012399,
%T 55555555342491359799151,1116817987709786226917069,578610396154837,
%U 66992050984853,254497141,1660738053545999,201525986561,25600818891233,796725607788661087,23547857117470471
%N a(0) = 7; for n > 0, a(n) is the greatest prime factor of PP(PP(a(n-1)))*a(n-1)+2 where PP(n) is an abbreviation for PreviousPrime(n).
%H Tyler Busby, <a href="/A082021/b082021.txt">Table of n, a(n) for n = 0..22</a>
%t NestWhileList[FactorInteger[2+#*Prime[PrimePi[ # ]-2]][[ -1,1]] &, 7, True, 8] (* _T. D. Noe_, Nov 15 2006 *)
%t NestList[FactorInteger[NextPrime[NextPrime[#,-1],-1]#+2][[-1,1]]&,7,20] (* _Harvey P. Dale_, Dec 26 2017 *)
%Y Cf. A082032, A031441, A031442.
%K nonn
%O 0,1
%A _Yasutoshi Kohmoto_, May 10 2003
%E Description corrected by _Rick L. Shepherd_, Dec 19 2004
%E Corrected by _T. D. Noe_, Nov 15 2006
%E More terms from _Harvey P. Dale_, Dec 26 2017
%E a(19) from _Tyler Busby_, Oct 12 2023