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

1, 5, 155, 353, 1144, 4297, 11921, 14027, 46169

Offset

1,2

Comments

Numbers k such that '393' appended to k times the digit 5 is prime.

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

a(10) > 60000. - Michael S. Branicky, Feb 18 2026

Links

Table of n, a(n) for n=1..9.

Example

1 appears because 5393 is prime.
5 appears because 55555393 ('5' concatenated 5 times and prepended to '393') is prime.

Maple

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

Mathematica

Select[Range[1500], PrimeQ[(2^(# + 3) * 5^(# + 4) - 1463) / 9] &]

Prog

(Magma) [n: n in [1..400] | IsPrime((2^(n+3) * 5^(n+4) - 1463) div 9)];
(PARI) is(n)=isprime((2^(n+3)*5^(n+4) - 1463)/9)

Crossrefs

Cf. A260903.

Sequence in context: A322634 A378842 A108535 * A237527 A015019 A225165

Adjacent sequences: A265120 A265121 A265122 * A265124 A265125 A265126

Keyword

nonn,base,hard,more

Author

Mikk Heidemaa, Dec 02 2015

Extensions

a(9) from Michael S. Branicky, Feb 18 2026

Status

approved