A127936 - OEIS

%I #27 Mar 23 2022 14:40:20

%S 1,2,3,5,6,8,9,11,15,21,30,39,50,63,83,95,99,156,173,350,854,1308,

%T 1769,2903,5250,5345,5639,6195,7239,21368,41669,47684,58619,63515,

%U 69468,70539,133508,134993,187160,493095

%N Numbers n such that 1 + Sum_{i=1..n} 2^(2i-1) is prime.

%C If this sequence is infinite then so is A124401.

%C Equals A127965(n)/2.

%C The sum has the simple closed form 1 + 2/3*(4^n-1). - _Stefan Steinerberger_, Nov 24 2007

%C Terms beyond a(30) correspond to probable primes, cf. A000978. - _M. F. Hasler_, Aug 29 2008

%F a(n) = floor(A000978(n)/2) = ceiling(log(4)(A000979(n))); A000978(n) = 2 a(n) + 1; A000979(n) = (2*4^a(n)+1)/3. - _M. F. Hasler_, Aug 29 2008

%e a(1)=1 because 1 + 2 = 3 is prime;

%e a(2)=2 because 1 + 2 + 2^3 = 11 is prime;

%e a(3)=3 because 1 + 2 + 2^3 + 2^5 = 43 is prime;

%e a(4)=5 because 1 + 2 + 2^3 + 2^5 + 2^7 + 2^9 = 683 is prime;

%e ...

%t a = {}; Do[If[PrimeQ[1 + Sum[2^(2n - 1), {n, 1, x}]], AppendTo[a, x]], {x, 1, 1000}]; a

%t b = {}; Do[c = 1 + Sum[2^(2n - 1), {n, 1, x}]; If[PrimeQ[c], AppendTo[b, c]], {x, 0, 1000}]; a = {}; Do[AppendTo[a, FromDigits[IntegerDigits[b[[x]], 2]]], {x, 1, Length[b]}]; d = {}; Do[AppendTo[d, (1/2)(DigitCount[a[[x]], 10, 0]+DigitCount[a[[x]], 10, 1]]), {x, 1, Length[a]}]; d

%t Position[Accumulate[2^(2*Range[1000]-1)],_?(PrimeQ[#+1]&)]//Flatten (* The program generates the first 21 terms of the sequence. To generate more, increase the Range constant. *) (* _Harvey P. Dale_, Mar 23 2022 *)

%o (PARI) for(n=1,999, ispseudoprime(2^(2*n+1)\3+1) & print1(n",")) \\ _M. F. Hasler_, Aug 29 2008

%o (Haskell)

%o import Data.List (findIndices)

%o a127936 n = a127936_list !! (n-1)

%o a127936_list = findIndices ((== 1) . a010051'') a007583_list

%o -- _Reinhard Zumkeller_, Mar 24 2013

%o (Python)

%o from sympy import isprime

%o A127936 = [i for i in range(1,10**3) if isprime(int('01'*i+'1',2))]

%o # _Chai Wah Wu_, Sep 05 2014

%Y Cf. A127962, A127963, A127964, A127965, A127961, A000979, A000978, A124400, A126614, A127955, A127956, A127957, A127958, A127936, A127936, A124401, A010051, A007583.

%K nonn,more

%O 1,2

%A _Artur Jasinski_, Feb 08 2007, Feb 09 2007

%E Edited by _N. J. A. Sloane_ at the suggestion of _Andrew S. Plewe_, Jun 11 2007

%E 2 more terms from _Stefan Steinerberger_, Nov 24 2007

%E 6 more terms from _Dmitry Kamenetsky_, Jul 12 2008

%E a(30)-a(40) calculated from A000978 by _M. F. Hasler_, Aug 29 2008