A118908 - OEIS

%I #9 Jun 16 2016 03:55:10

%S 4,15,221,48839,2385247913,5689407606470855563,

%T 32369358912568429679140929317208046943,

%U 1047775396410673232345014594095988998399127191704501568910205139392491645247

%N a(1) = 4; a(n) is greatest semiprime < a(n-1)^2.

%C Semiprime analog of A059785 a(n+1)=prevprime(a(n)^2), with a(1) = 2. With that, of course, there's always a prime between n and 2n, so a(n) < 2^n. See also A055496 a(1) = 2; a(n) is smallest prime > 2*a(n-1). The obverse of this is A118909 a(1) = 4; a(n) is least semiprime > a(n-1)^2.

%C a(9), which is too large to be included, is equal to a(8)^2-3. - _Giovanni Resta_, Jun 16 2016

%e a(6) = 32369358912568429679140929317208046943 = 1816568472934912211 * 17818958874845686213 = 5689407606470855563^2 - 26 < a(5)^2.

%Y Cf. A001358, A055496, A076656, A006992, A005384, A005385, A118908-A118913.

%K easy,nonn

%O 1,1

%A _Jonathan Vos Post_, May 05 2006