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A175282 Positive numbers n with property that n^2+3n+9 is prime (A005471). 4


%S 1,2,4,7,8,10,11,16,17,23,25,28,29,31,32,37,38,43,49,50,56,58,64,70,

%T 73,85,88,91,94,95,98,101,107,112,121,122,127,130,133,134,136,140,142,

%U 143,155,158,163,164,169,172,175,176,179,182,197,200,205,206,212,214,218

%N Positive numbers n with property that n^2+3n+9 is prime (A005471).

%C Notice that at n=-1, n^2+3n+9=7 is also (positive) prime.

%H Erik Dofs, <a href="http://pldml.icm.edu.pl/pldml/element/bwmeta1.element.bwnjournal-number-aa-1995-73-3">Solutions of x^3+y^3+z^3=n*x*y*z</a>, Acta Arithm. 73.3 (1995) 201

%p A175282 := proc(n)

%p option remember;

%p if n = 1 then

%p 1;

%p else

%p for a from procname(n-1)+1 do

%p if isprime(a^2+3*a+9) then

%p return a;

%p end if;

%p end do;

%p end if;

%p end proc: # _R. J. Mathar_, Jun 06 2019

%t Select[Range[1,400],PrimeQ[ #^2+3*#+9]&]

%Y Cf. A005471, A175281.

%K nonn

%O 1,2

%A _Zak Seidov_, Mar 21 2010

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Last modified May 7 12:56 EDT 2021. Contains 343650 sequences. (Running on oeis4.)