OFFSET
1,1
COMMENTS
Prime numbers n for which cos(2*Pi/n) is an algebraic number of 7th degree. - Artur Jasinski, Dec 13 2006
LINKS
Harry J. Smith and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 500 terms from Smith)
FORMULA
Primes of form 2^a*3^b*5^c*7^d + 1 with a and d > 1.
EXAMPLE
For n = {4, 8, 9, 12}, a(n)-1 = {70, 210, 280, 420} = 7*{10, 30, 40, 60}.
MATHEMATICA
Select[Prime[Range[2000]], FactorInteger[#-1][[-1, 1]] ==7&] (* Harvey P. Dale, Mar 12 2011 *)
PROG
(PARI) default(primelimit, 108864001); n=0; forprime (p=3, 108864001, f=factor(p - 1)~; if (f[1, length(f)]==7, write("b061638.txt", n++, " ", p))) \\ Harry J. Smith, Jul 25 2009
(PARI) list(lim)=my(v=List(), t, t5, t7); lim\=1; lim--; for(a=1, logint(lim\2, 7), t7=2*7^a; for(b=0, logint(lim\t7, 5), t5=5^b*t7; for(c=0, logint(lim\t5, 3), t=3^c*t5; while(t<=lim, if(isprime(t+1), listput(v, t+1)); t<<=1)))); Set(v) \\ Charles R Greathouse IV, Oct 29 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jun 13 2001
STATUS
approved