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
KEYWORD
nonn,base
AUTHOR
Parthasarathy Nambi, Aug 30 2005
EXTENSIONS
More terms from Vincenzo Librandi, Mar 27 2010
STATUS
approved