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A266421 Numbers n such that (2^(n+7)*5^(n+5) - 204979)/9 is prime (n > 0). 3
5, 11, 127, 149, 199, 239, 503, 1069, 4073 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers n such that '21669' appended to n times the digit 4 is prime.

Up to a(9) the terms themselves are primes.

a(1), a(2), a(6), a(9), and (2^(a(1)+7) * 5^(a(1)+5) - 204979)/9 = 4444421669 are also Sophie Germain primes.

a(10) > 50000 (if it exists).

LINKS

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

EXAMPLE

5 appears because 4444421669 ('4' concatenated 5 times and prepended to '21669') is prime.

MAPLE

A266421:=n->`if`(isprime((2^(n+7) * 5^(n+5) - 204979)/9), n, NULL): seq(A266421(n), n=1..5000);

MATHEMATICA

Select[ Range[5000], PrimeQ[(2^(# + 7) * 5^(# + 5) - 204979)/9] &]

PROG

(MAGMA)[n: n in[1 .. 1000] | IsPrime((2^(n+7) * 5^(n+5) - 204979) div 9)];

(PARI) is(n)=isprime((2^(n+7) * 5^(n+5) - 204979)/9)

CROSSREFS

Cf. A260903, A265629.

Sequence in context: A222568 A248253 A262309 * A094108 A222674 A197538

Adjacent sequences:  A266418 A266419 A266420 * A266422 A266423 A266424

KEYWORD

nonn,base,hard,more

AUTHOR

Mikk Heidemaa, Dec 29 2015

STATUS

approved

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Last modified October 20 12:47 EDT 2019. Contains 328257 sequences. (Running on oeis4.)