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 A265123 Numbers n such that (2^(n+3) * 5^(n+4) - 1463)/9 is prime (n > 0). 1

%I

%S 1,5,155,353,1144,4297,11921,14027

%N Numbers n such that (2^(n+3) * 5^(n+4) - 1463)/9 is prime (n > 0).

%C Numbers n such that '393' appended to n times the digit 5 is prime.

%C Up to a(7) nonprimes alternate with primes; a(9) > 30000 (if it exists).

%e 1 appears because 5393 is prime.

%e 5 appears because 55555393 ('5' concatenated 5 times and prepended to '393') is prime.

%p A265123:=n->`if`(isprime((2^(n+3) * 5^(n+4) - 1463)/9), n, NULL): seq(A265123(n), n=1..1500);

%t Select[Range, PrimeQ[(2^(# + 3) * 5^(# + 4) - 1463) / 9] &]

%o (MAGMA) [n: n in [1..400] | IsPrime((2^(n+3) * 5^(n+4) - 1463) div 9)];

%o (PARI) is(n)=isprime((2^(n+3)*5^(n+4) - 1463)/9)

%Y Cf. A260903.

%K nonn,base,hard,more

%O 1,2

%A _Mikk Heidemaa_, Dec 02 2015

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Last modified May 18 22:37 EDT 2022. Contains 353826 sequences. (Running on oeis4.)