login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A087606 Smallest k such that n times concatenation of k with itself followed by a 9 is a prime, or 0 if no such number exists. 7
1, 2, 0, 1, 1, 0, 1, 11, 0, 64, 5, 0, 2, 31, 0, 1, 5, 0, 10, 65, 0, 41, 212, 0, 5, 79, 0, 41, 160, 0, 5, 94, 0, 8, 82, 0, 23, 43, 0, 40, 26, 0, 391, 119, 0, 212, 4, 0, 1, 160, 0, 134, 28, 0, 208, 50, 0, 248, 35, 0, 113, 43, 0, 79, 7, 0, 70, 170, 0, 64, 94, 0, 19, 86, 0, 10, 118, 0, 34, 98 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Conjecture: a(3n) = 0. No other term is zero.

a(3n)=0: consider the sum of the digits modulo 3. For the same reason, if a(m) is divisible by 3 then a(m)=0. - Sam Alexander, Nov 15 2003

LINKS

Table of n, a(n) for n=1..80.

XIAO Gang, Factoris - a program that factorizes huge integers, 1997-1999

EXAMPLE

a(2) = 2 as 229 is a prime. but 119 is not.

MATHEMATICA

s[b_]:=(v={}; l=Length[b]; Do[v=Join[v, IntegerDigits[b[[k]]]], {k, l}]; v); a[n_]:=If[Mod[n, 3]!= 0, (For[m = 1, ! PrimeQ[10*FromDigits[s[Table[m, {n}]]] +9], m++ ]; m), 0]; Table[a[n], {n, 90}] (Firoozbakht)

CROSSREFS

Cf. A086920, A087604, A087605, A087607, A087608, A087609, A087610.

Sequence in context: A083731 A216266 A177416 * A271368 A116799 A057556

Adjacent sequences:  A087603 A087604 A087605 * A087607 A087608 A087609

KEYWORD

base,nonn

AUTHOR

Amarnath Murthy, Sep 18 2003

EXTENSIONS

More terms from Sam Alexander, Nov 15 2003

More terms from Farideh Firoozbakht, Feb 04 2005

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
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 20 20:08 EDT 2021. Contains 345231 sequences. (Running on oeis4.)