OFFSET
1,1
COMMENTS
Alternative definition: increasing list of primes p such that for ALL primes q, every residue mod p is the sum of two q-th powers mod p (if q does not divides p-1 then every residue mod p is a q-th power, so only the case q divides p-1 is not trivial).
Related to the conjecture:
For every prime q there are only finitely many primes p such that not every residue mod p is the sum of two q-th powers mod p.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..400
EXAMPLE
p=7 is not in the list because q=3 divides p-1 and the only residues mod 7 which are the sum of two cubic residues mod 7 are: 0,1,-1,2,-2.
To check that p=13 is in the sequence, since 2 and 3 are the only primes dividing p-1=12, we only need to see that every residue mod 13 is both the sum of two quadratic residues mod 13 and the sum of two cubic residues mod 13.
PROG
(Sage)
for p in prime_range(a, b):
c=1
C=GF(p)
u=combinations_with_replacement(C, 2)
v=[x for x in u]
for q in prime_divisors(p-1):
w=(k[0]^q+k[1]^q for k in v)
s=set(w)
l=len(s)
if l!=p:
c=0
break
if c==1:
print(p)
(PARI) is(p)=if(!isprime(p), return(0)); my(f=factor(p-1)[, 1], v, u); for(i=1, #f, u=vector(p); v=vector(p, j, j^f[i]%p); for(j=1, p, for(k=j, p, u[(v[j]+v[k])%p+1]=1)); if(!vecmin(u), return(0))); 1 \\ Charles R Greathouse IV, Jan 23 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Esteban Arreaga Ambéliz, Jan 19 2014
STATUS
approved