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A058383 Primes of form 1+(2^a)*(3^b), a>0, b>0. 18
7, 13, 19, 37, 73, 97, 109, 163, 193, 433, 487, 577, 769, 1153, 1297, 1459, 2593, 2917, 3457, 3889, 10369, 12289, 17497, 18433, 39367, 52489, 139969, 147457, 209953, 331777, 472393, 629857, 746497, 786433, 839809, 995329, 1179649, 1492993 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Prime numbers n such that cos(2*Pi/n) is an algebraic number of a 3-smooth degree, but not a 2-smooth degree. - Artur Jasinski, Dec 13 2006

From Antonio M. Oller-Marcén, Sep 24 2009: (Start)

In this case gcd(a,b) is a power of 2.

A regular polygon of n sides is constructible by paper folding if and only if n=2^r3^sp_1...p_t with p_i being distinct primes of this kind. (End)

Primes in A005109 but not in A092506. - R. J. Mathar, Sep 28 2012

Conjecture: these are the only solutions >=7 to the equation A000010(x) + A000010(x-1) = floor((4*x-3)/3). - Benoit Cloitre, Mar 02 2018

These are also called Pierpont primes. - Harvey P. Dale, Apr 13 2019

LINKS

Ray Chandler, Table of n, a(n) for n = 1..8378 (terms < 10^1000, first 1000 terms from T. D. Noe)

FORMULA

Primes of the form 1 + A033845(n).

MAPLE

N:= 10^10: # to get all terms <= N+1

sort(select(isprime, [seq(seq(1+2^a*3^b, a=1..ilog2(N/3^b)), b=1..floor(log[3](N)))])); # Robert Israel, Mar 02 2018

MATHEMATICA

Do[If[Take[FactorInteger[EulerPhi[2n + 1]][[ -1]], 1] == {3} && PrimeQ[2n + 1], Print[2n + 1]], {n, 1, 10000}] - Artur Jasinski, Dec 13 2006

mx = 1500000; s = Sort@ Flatten@ Table[1 + 2^j*3^k, {j, Log[2, mx]}, {k, Log[3, mx/2^j]}]; Select[s, PrimeQ] (* Robert G. Wilson v, Sep 28 2012 *)

Select[Prime[Range[114000]], FactorInteger[#-1][[All, 1]]=={2, 3}&] (* Harvey P. Dale, Apr 13 2019 *)

CROSSREFS

Cf. A033845, A000423, A125866, A217035.

Sequence in context: A176229 A266268 A110074 * A005471 A249381 A040096

Adjacent sequences:  A058380 A058381 A058382 * A058384 A058385 A058386

KEYWORD

nonn

AUTHOR

Labos Elemer, Dec 20 2000

STATUS

approved

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Last modified February 25 19:53 EST 2020. Contains 332258 sequences. (Running on oeis4.)