A219041 Numbers k such that 3^k - 20 is prime.

3, 4, 5, 6, 10, 11, 19, 20, 23, 25, 26, 71, 80, 91, 101, 139, 150, 179, 200, 246, 599, 626, 1126, 2215, 4189, 7795, 30626, 66941, 87630, 104388

Offset

1,1

Comments

a(31) > 2*10^5. - Robert Price, Nov 14 2013

Links

Table of n, a(n) for n=1..30.

Example

3^3 - 20 = 7 (prime), so 3 is in the sequence.

Mathematica

Do[If[PrimeQ[3^n - 20], Print[n]], {n, 3, 10000}]

Prog

(PARI) is(n)=isprime(3^n-20) \\ Charles R Greathouse IV, Feb 17 2017

Crossrefs

Cf. Sequences of numbers k such that 3^k + m is prime:

(m = 2) A051783, (m = -2) A014224, (m = 4) A058958, (m = -4) A058959,

(m = 8) A217136, (m = -8) A217135, (m = 10) A217137, (m = -10) A217347,

(m = 14) A219035, (m = -14) A219038, (m = 16) A205647, (m = -16) A219039,

(m = 20) A219040, (m = -20) A219041, (m = 22) A219042, (m = -22) A219043,

(m = 26) A219044, (m = -26) A219045, (m = 28) A219046, (m = -28) A219047,

(m = 32) A219048, (m = -32) A219049, (m = 34) A219050, (m = -34) A219051. Note that if m is a multiple of 3, 3^k + m is also a multiple of 3 (for k greater than 0), and as such isn't prime.

Sequence in context: A138918 A071186 A263118 * A218946 A174057 A335231

Adjacent sequences: A219038 A219039 A219040 * A219042 A219043 A219044

Keyword

nonn,more

Author

Nicolas M. Perrault, Nov 10 2012

Extensions

a(27)-a(30) from Robert Price, Nov 14 2013

Status

approved