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A118908 a(1) = 4; a(n) is greatest semiprime < a(n-1)^2. 5
4, 15, 221, 48839, 2385247913, 5689407606470855563, 32369358912568429679140929317208046943, 1047775396410673232345014594095988998399127191704501568910205139392491645247 (list; graph; refs; listen; history; text; internal format)
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: A137654 A277509 A195569 * A153060 A139244 A090115

Adjacent sequences:  A118905 A118906 A118907 * A118909 A118910 A118911

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post, May 05 2006

STATUS

approved

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Last modified June 25 23:52 EDT 2019. Contains 324367 sequences. (Running on oeis4.)