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%I #20 Jul 07 2024 17:24:31
%S 11,31,41,61,101,151,181,241,251,271,401,541,601,641,751,811,1201,
%T 1601,1621,1801,2161,2251,3001,4001,4051,4801,4861,6481,7681,8101,
%U 8641,9001,9601,9721,11251,14401,15361,16001,19441,21601,21871,22501,23041,24001
%N Primes p such that the greatest prime divisor of p-1 is 5.
%C Prime numbers n for which cos(2Pi/n) is an algebraic number of 5th degree. - _Artur Jasinski_, Dec 13 2006
%C The least significant digit of each term is one. - _Harvey P. Dale_, Jul 07 2024
%H Charles R Greathouse IV, <a href="/A061599/b061599.txt">Table of n, a(n) for n = 1..10000</a>
%F Primes of the form 2^a*3^b*5^c + 1 with a and c > 0.
%t Do[If[Take[FactorInteger[EulerPhi[2n + 1]][[ -1]],1] == {5} && PrimeQ[2n + 1], Print[2n + 1]], {n, 1, 10000}] (* _Artur Jasinski_, Dec 13 2006 *)
%t Select[Prime[Range[3000]],Max[FactorInteger[#-1][[;;,1]]]==5&] (* _Harvey P. Dale_, Jul 07 2024 *)
%o (PARI) { default(primelimit, 167772161); n=0; forprime (p=3, 167772161, f=factor(p - 1)~; if (f[1, length(f)]==5, write("b061599.txt", n++, " ", p)) ) } \\ _Harry J. Smith_, Jul 25 2009
%o (PARI) list(lim)=my(v=List(), s, t); lim\=1; lim--; for(i=1, logint(lim\2, 5), t=2*5^i; for(j=0, logint(lim\t, 3), s=t*3^j; while(s<=lim, if(isprime(s+1), listput(v, s+1)); s<<=1))); Set(v) \\ _Charles R Greathouse IV_, Oct 29 2018
%Y The 3rd in a family of sequences after A019434(=Fermat-primes) and A058383.
%Y Cf. A019434, A058383, A023503, A034694, A006530, A006093, A035095, A000040.
%Y Cf. A004729, A058383, A125867-A125875, A024899.
%K nonn
%O 1,1
%A _Labos Elemer_, Jun 13 2001
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