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A061242 Primes of the form 9*k - 1. 15
17, 53, 71, 89, 107, 179, 197, 233, 251, 269, 359, 431, 449, 467, 503, 521, 557, 593, 647, 683, 701, 719, 773, 809, 827, 863, 881, 953, 971, 1061, 1097, 1151, 1187, 1223, 1259, 1277, 1367, 1439, 1493, 1511, 1583, 1601, 1619, 1637, 1709, 1871, 1889, 1907 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Or, primes of the form 18i-1. Corresponding values of i are in A138918. - Zak Seidov, Apr 03 2008

A010888(a(n)) = 8. - Reinhard Zumkeller, Feb 25 2005

From Doug Bell, Mar 23 2009: (Start)

It is my conjecture that if a(n) = 9x-1, the integer formed by the repeating digits in the decimal fraction x/a(n) is the smallest integer such that rotating the digits to the left produces a number which is ((x+1)/x) times larger.

Example: x=2, a(n) = 17: 2/17 = 0.1176470588235294... repeating with a cycle of 16.

1176470588235294 x (3/2) = 1764705882352941, which is 1176470588235294 rotated to the left.

An additional conjecture is that the values of x from this sequence are the only values where rotating an integer one to the left produces a value (x+1)/x times as large. (End)

The last conjecture is false. For example, for x=3 we have 230769*(4/3)=307692, but 9*3-1 = 26 is not in the sequence. - Giovanni Resta, Jul 28 2015

Conjecture: Primes p such that ((x+1)^9-1)/x has 4 irreducible factors of degree 2 over GF(p). - Federico Provvedi, Jun 27 2018

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..1000

MAPLE

select(isprime, [seq(18*i-1, i=1..1000)]); # Robert Israel, Sep 03 2014

MATHEMATICA

Select[ Range[ 2500 ], PrimeQ[ # ] && Mod[ #, 9 ] == 8 & ]

Select[9*Range[300] - 1, PrimeQ]

PROG

(Python)

from sympy import prime

A061242 = [p for p in (prime(n) for n in range(1, 10**3)) if not (p+1) % 18]

# Chai Wah Wu, Sep 02 2014

(MAGMA) [a: n in [0..250] | IsPrime(a) where a is 9*n - 1 ]; // Vincenzo Librandi, Jun 07 2015

CROSSREFS

Cf. A061237, A061238, A061239, A061240, A061241, A138918, A258663.

Sequence in context: A213997 A062342 A295869 * A062343 A176254 A287311

Adjacent sequences:  A061239 A061240 A061241 * A061243 A061244 A061245

KEYWORD

nonn,easy

AUTHOR

Amarnath Murthy, Apr 23 2001

EXTENSIONS

More terms from Robert G. Wilson v, May 10 2001

Edited by N. J. A. Sloane at the suggestion of R. J. Mathar, Apr 30 2008

STATUS

approved

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Last modified August 7 17:44 EDT 2020. Contains 336278 sequences. (Running on oeis4.)