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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
OFFSET
1,1
LINKS
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: A102167 A214525 A372539 * A039513 A125265 A023152
KEYWORD
nonn,base
AUTHOR
Parthasarathy Nambi, Aug 30 2005
EXTENSIONS
More terms from Vincenzo Librandi, Mar 27 2010
STATUS
approved