The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A272232 Smallest k > 0 such that R_k//n//R_k is prime, where R_k is the repunit A002275(k) and // denotes concatenation; or 0 if no such k exists. 1
 1, 9, 0, 1, 2, 1, 10, 3, 1, 1, 3, 0, 2, 3, 33, 1, 2, 1, 1, 21, 1, 2, 0, 1, 7, 48, 292, 4, 3, 1, 1, 2, 1, 0, 135, 0, 1, 0, 1, 34, 3, 3, 40, 2, 0, 1, 3, 1, 1, 32, 61, 1, 2, 1, 137, 0, 3, 1, 2, 42, 1, 14, 1, 262, 2, 22, 0, 3, 9, 2, 33, 73, 1, 3, 1, 2, 3, 0, 2, 2, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(2) = 0 (see second comment in A258372). a(n) = 0 if n > 0 is in A099814 (see fourth comment in A004022). a(n) = 0 if n is of the form A000042(i)*10^j+A000042(i) for some j > i > 0, since the resulting number is divisible by A002275(k)//A000042(i). a(n) = 0 if n is a term of A010785 with an even number of digits, since any number of the form 1..1d..d1..1 with an even number of digits d is divisible by 11. a(n) = 1 if there exists an integer x such that n = (A002275(A004023(x))-A011557(x)-1)/10. From Chai Wah Wu, Nov 07 2019: (Start) a(n) = 0 if n has an even number of digits and is a multiple of 11. In particular, a(n) = 0 if n is a term of A056524. a(n) = 0 if n = (10^k+1)(10^m-1)/9 for some m > 0, k >= 0. (End) LINKS EXAMPLE a(0) = 1 since 101 is prime; a(1) refers to the prime 1111111111111111111. MATHEMATICA Table[SelectFirst[Range[10^4], PrimeQ@ FromDigits@ Flatten@ {#, IntegerDigits@ n, #} &@ Table[1, {#}] &], {n, 0, 91}] /. k_ /; MissingQ@ k -> 0 (* Michael De Vlieger, Apr 25 2016, Version 10.2 *) PROG (PARI) a(n) = my(k=1); while(!ispseudoprime(eval(Str((10^k-1)/9, n, (10^k-1)/9))), k++); k CROSSREFS Cf. A000042, A002275, A056524, A098814, A004023, A258372. Sequence in context: A274186 A158336 A021530 * A110909 A197070 A197333 Adjacent sequences:  A272229 A272230 A272231 * A272233 A272234 A272235 KEYWORD nonn,base AUTHOR Felix Fröhlich, Apr 23 2016 EXTENSIONS a(35)-a(80) from Giovanni Resta, May 01 2016 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.

Last modified March 30 19:28 EDT 2020. Contains 333127 sequences. (Running on oeis4.)