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A028416 Primes p such that the decimal expansion of 1/p has a periodic part of even length. 10
7, 11, 13, 17, 19, 23, 29, 47, 59, 61, 73, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 157, 167, 179, 181, 193, 197, 211, 223, 229, 233, 241, 251, 257, 263, 269, 281, 293, 313, 331, 337, 349, 353, 367, 373, 379, 383, 389, 401, 409, 419, 421, 433 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Primes whose reciprocals have even period length.

Primes p such that the order of 10 mod p is even. [Joerg Arndt, Mar 04 2014]

A002371(A049084(a(n))) mod 2 == 0.

Not the same as A040121: a(33)=241 is not in A040121.

Let (d(i): 1<=i<=2*K) be the period of decimal expansion of 1/a(n), K=A002371(A049084(a(n)))/2, then d(i) + d(i+K) = 9 for i with 1<=i<=K, or, equivalently: u + v = 10^K - 1 with u = SUM(d(i)*10^(K-i):1<=i<=K) and v = SUM(d(i+K)*10^(K-i):1<=i<=K). - Reinhard Zumkeller, Oct 05 2008

REFERENCES

H. Rademacher and O. Toeplitz, Von Zahlen und Figuren (Springer 1930, reprinted 1968), ch. 19, 'Die periodischen Dezimalbrueche'. [Reinhard Zumkeller, Oct 05 2008]

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

Index entries for sequences related to decimal expansion of 1/n.

EXAMPLE

From Reinhard Zumkeller, Oct 05 2008: (Start)

(0,5,8,8,2,3,5,2,9,4,1,1,7,6,4,7) is the period of 1/17 (see A007450),

K = A002371(A049084(17))/2 = A002371(7)/2 = 16/2 = 8,

u = 5882352, v = 94117647: u + v = 99999999 = 10^8 - 1. (End)

MAPLE

A028416 := proc(n) local st:

st := ithprime(n):

if (modp(numtheory[order](10, st), 2) = 0) then

   RETURN(st)

fi: end:  seq(A028416(n), n=1..100); # Jani Melik, Feb 24 2011

MATHEMATICA

Select[Prime[Range[4, 100]], EvenQ[Length[RealDigits[1/#][[1, 1]]]]&] (* Harvey P. Dale, Jul 07 2011 *)

PROG

(PARI) forprime(p=7, 1e3, if(znorder(Mod(10, p))%2==0, print1(p", "))) \\ Charles R Greathouse IV, Feb 24 2011

CROSSREFS

Cf. A087000.

Sequence in context: A135776 A067831 A086998 * A040121 A156114 A304690

Adjacent sequences:  A028413 A028414 A028415 * A028417 A028418 A028419

KEYWORD

nonn,base

AUTHOR

Mario Velucchi (mathchess(AT)velucchi.it)

EXTENSIONS

More terms from Reinhard Zumkeller, Jul 29 2003

STATUS

approved

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Last modified April 5 20:30 EDT 2020. Contains 333260 sequences. (Running on oeis4.)