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A128066 Numbers k such that (3^k + 4^k)/7 is prime. 17
3, 5, 19, 37, 173, 211, 227, 619, 977, 1237, 2437, 5741, 13463, 23929, 81223, 121271 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All terms are primes.

LINKS

Table of n, a(n) for n=1..16.

Henri Lifchitz & Renaud Lifchitz, Top probable primes of the form (4^p+3^p)/7

MAPLE

a:=proc(n) if type((3^n+4^n)/7, integer)=true and isprime((3^n+4^n)/7)=true then n else fi end: seq(a(n), n=1..1500); # Emeric Deutsch, Feb 17 2007

MATHEMATICA

Do[ p=Prime[n]; f=(3^p+4^p)/(4+3); If[ PrimeQ[f], Print[p]], {n, 1, 100} ]

PROG

(PARI) f(n)=(3^n + 4^n)/7;

forprime(n=3, 10^5, if(ispseudoprime(f(n)), print1(n, ", ")))

/* Joerg Arndt, Mar 27 2011 */

CROSSREFS

Cf. A007658 = n such that (3^n + 1)/4 is prime; A057469 ((3^n + 2^n)/5); A122853 ((3^n + 5^n)/8).

Cf. A128067, A128068, A128069, A128070, A128071, A128072, A128073, A128074, A128075.

Cf. A059801 (4^n - 3^n); A121877 ((5^n - 3^n)/2).

Cf. A128024, A128025, A128026, A128027, A128028, A128029, A128030, A128031, A128032.

Sequence in context: A228471 A062594 A128027 * A273020 A148523 A148524

Adjacent sequences:  A128063 A128064 A128065 * A128067 A128068 A128069

KEYWORD

hard,more,nonn

AUTHOR

Alexander Adamchuk, Feb 14 2007

EXTENSIONS

3 more terms from Emeric Deutsch, Feb 17 2007

2 more terms from Farideh Firoozbakht, Apr 16 2007

Two more terms (13463 and 23929) found by Lelio R Paula in 2008 corresponding to probable primes with 8105 and 14406 digits. Jean-Louis Charton, Oct 06 2010

Two more terms (81223 and 121271) found by Jean-Louis Charton in March 2011 corresponding to probable primes with 48901 and 73012 digits

STATUS

approved

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Last modified October 23 08:57 EDT 2019. Contains 328345 sequences. (Running on oeis4.)