A118908 a(1) = 4; a(n) is greatest semiprime < a(n-1)^2.

4, 15, 221, 48839, 2385247913, 5689407606470855563, 32369358912568429679140929317208046943, 1047775396410673232345014594095988998399127191704501568910205139392491645247

Offset

1,1

Comments

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.

a(9), which is too large to be included, is equal to a(8)^2-3. - Giovanni Resta, Jun 16 2016

Links

Table of n, a(n) for n=1..8.

Example

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

Crossrefs

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

Sequence in context: A349402 A277509 A195569 * A153060 A139244 A369727

Adjacent sequences: A118905 A118906 A118907 * A118909 A118910 A118911

Keyword

easy,nonn

Author

Jonathan Vos Post, May 05 2006

Status

approved