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.

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.

Links

Table of n, a(n) for n=1..29.

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

Adjacent sequences: A088118 A088119 A088120 * A088122 A088123 A088124

Keyword

easy,nonn

Author

Amarnath Murthy, Sep 25 2003

Extensions

More terms from David Wasserman, Jul 25 2005

Status

approved