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A128344 Numbers k such that (7^k - 5^k)/2 is prime. 20
3, 5, 7, 113, 397, 577, 7573, 14561, 58543 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All terms are primes.

No other terms less than 10^5. - Robert Price, May 28 2012

LINKS

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

MATHEMATICA

k=7; Do[p=Prime[n]; f=(k^p-5^p)/(k-5); If[ PrimeQ[f], Print[p] ], {n, 1, 100}]

PROG

(PARI) is(n)=isprime((7^n-5^n)/2) \\ Charles R Greathouse IV, Feb 17 2017

CROSSREFS

Cf. A062572, A128345, A128346, A128347, A128348, A128349, A128350, A128351, A128352, A128353, A128354. Cf. A004061, A082182, A121877, A059802. Cf. A057171, A082387, A122853, A128335, A128336, A128337, A128338, A128339, A128340, A128341, A128342.

Sequence in context: A132287 A046201 A292848 * A259385 A114366 A088092

Adjacent sequences:  A128341 A128342 A128343 * A128345 A128346 A128347

KEYWORD

hard,more,nonn

AUTHOR

Alexander Adamchuk, Feb 27 2007

EXTENSIONS

a(7)-a(9) from Robert Price, May 28 2012

STATUS

approved

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Last modified May 26 07:44 EDT 2019. Contains 323579 sequences. (Running on oeis4.)