login
A088121
Smallest prime obtained as a sum of n terms of a geometric progression + the common ratio, or 0 if no such terms exists. Smallest prime of the form (a +ar +ar^2 + ar^3 +... ) + r.
1
3, 5, 23, 17, 157, 191, 383, 257, 2557, 9209, 6143, 20477, 73721, 147449, 360439, 65537, 655357, 786431, 11010029, 5242877, 31457267, 71303153, 276824033, 150994937, 301989881, 469762043, 671088637, 4026531827, 2684354557
OFFSET
1,1
COMMENTS
The ratio is required to be > 1.
For a(5), (1 + r + r^2 + r^3 + r^4) + r is composite. hence the first term of the geometric progression is >1. For a(5) a = 5 and r = 2. This is true For all odd n.
EXAMPLE
a(3) = (3+6+12) + 2 = 23.
a(4) = (1+2+4+8 ) + 2 = 17.
a(6) = (3+6+12+24+48+96) + 2= 191.
CROSSREFS
Cf. A088120.
Sequence in context: A270793 A270209 A271464 * A144103 A270155 A271308
KEYWORD
easy,nonn
AUTHOR
Amarnath Murthy, Sep 25 2003
EXTENSIONS
More terms from David Wasserman, Jul 25 2005
STATUS
approved