The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A109637 Numbers n such that the string 55n is prime. 1
 7, 19, 21, 27, 31, 57, 63, 69, 73, 81, 91, 103, 109, 117, 127, 147, 163, 171, 201, 207, 213, 217, 219, 229, 243, 249, 259, 291, 313, 331, 333, 337, 339, 343, 351, 373, 381, 399, 411, 439, 441, 457, 469, 487, 501, 511, 529, 541, 547, 579, 589, 603, 609, 619 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..2000 EXAMPLE If n=7 then 55n = 557 which is a prime. If n=81 then 55n = 5581 which is a prime. MAPLE a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))       while not isprime(parse(""||55||k)) do od; a(n):=k     end: seq(a(n), n=1..100);  # Alois P. Heinz, Jul 30 2014 MATHEMATICA Select[Range[1, 1001, 2], PrimeQ[FromDigits[Join[{5, 5}, IntegerDigits[#]]]]&] (* Harvey P. Dale, Dec 02 2010 *) Select[Range[700], PrimeQ[55 10^IntegerLength[#] + #] &] (* Vincenzo Librandi, Jul 30 2014 *) PROG (MAGMA) [n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [5, 5])) ]; // Vincenzo Librandi, Jul 30 2014 CROSSREFS Sequence in context: A281915 A102167 A214525 * A039513 A125265 A023152 Adjacent sequences:  A109634 A109635 A109636 * A109638 A109639 A109640 KEYWORD nonn,base AUTHOR Parthasarathy Nambi, Aug 30 2005 EXTENSIONS More terms from Vincenzo Librandi, Mar 27 2010 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 January 22 12:55 EST 2022. Contains 350481 sequences. (Running on oeis4.)