OFFSET
1,9
COMMENTS
Conjecture: (i) a(n) > 0 for all n > 7.
(ii) Any integer n > 9 can be written as k + m with k > 1 and m > 1 such that the least nonnegative residue of prime(k) modulo k and the least nonnegative residue of prime(m) modulo m are both prime.
We have verified a(n) > 0 for all n = 8, ..., 10^8.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..10000
EXAMPLE
a(11) = 1 since 11 = 2 + 9, prime(2) = 3 == 1^2 (mod 2), and prime(9) = 23 == 5 (mod 9) with 5 prime.
a(16) = 1 since 16 = 12 + 4, prime(12) = 37 == 1^2 (mod 12), and prime(4) = 7 == 3 (mod 4) with 3 prime.
MATHEMATICA
SQ[n_]:=IntegerQ[Sqrt[n]]
s[k_]:=SQ[Mod[Prime[k], k]]
p[k_]:=PrimeQ[Mod[Prime[k], k]]
a[n_]:=Sum[Boole[s[k]&&p[n-k]], {k, 2, n-2}]
Table[a[n], {n, 1, 80}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, May 27 2014
STATUS
approved