login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A049479 Smallest prime dividing 2^n - 1. 12
3, 7, 3, 31, 3, 127, 3, 7, 3, 23, 3, 8191, 3, 7, 3, 131071, 3, 524287, 3, 7, 3, 47, 3, 31, 3, 7, 3, 233, 3, 2147483647, 3, 7, 3, 31, 3, 223, 3, 7, 3, 13367, 3, 431, 3, 7, 3, 2351, 3, 127, 3, 7, 3, 6361, 3, 23, 3, 7, 3, 179951, 3, 2305843009213693951, 3, 7, 3, 31 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

If p is prime then a(p) == 1 (mod p). Are there composite numbers n such that a(n) == 1 (mod n)? - Thomas Ordowski, Jan 27 2014

Yes, up to 1200, the following composites have the desired property: 169, 221, 323, 611, 779, 793, 923, 1121, 1159. - Michel Marcus, Jan 28 2014

a(n) <= a(lpf(n)) for every n, where lpf(n) = A020639(n). For which n is a(n) < a(lpf(n))? See: A236769. - Thomas Ordowski, Jan 30 2014

LINKS

Michel Marcus, Table of n, a(n) for n = 2..1060 (terms 2..500 from T. D. Noe)

Eric Weisstein's World of Mathematics, Mersenne Number

FORMULA

a(n) > lpf(n) while a(2k) = 3 and a(2k+1) > 2*lpf(2k+1), where lpf(m) = A020639(m). - Thomas Ordowski, Jan 29 2014

For k >= 1, a(2k) = 3, a(6k-3)=7. - Zak Seidov, Mar 21 2014

EXAMPLE

a(6)=3 since 2^6 - 1 = 63 = 3^2*7.

MATHEMATICA

a = {}; Do[w = 2^n - 1; c = FactorInteger[w]; b = c[[1]][[1]]; AppendTo[a, b], {n, 2, 65}]; a (* Artur Jasinski, Dec 11 2007 *)

CROSSREFS

Cf. A005420.

Sequence in context: A282160 A019158 A086153 * A125314 A213244 A050393

Adjacent sequences:  A049476 A049477 A049478 * A049480 A049481 A049482

KEYWORD

nonn

AUTHOR

Olivier Gérard

EXTENSIONS

More terms from Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 22 21:09 EDT 2018. Contains 316505 sequences. (Running on oeis4.)