OFFSET
0,3
EXAMPLE
a(2)=2 because 10^2 = 9 + 91 = 49 + 51, 9 and 91 are composite and coprime, 49 and 51 are composite and coprime.
MATHEMATICA
a[n_] := Module[{cnt=0}, Do[If[!PrimeQ[k]&&!PrimeQ[10^n-k]&&GCD[k, 10^n-k]==1, cnt++], {k, 3, (1/2)10^n, 2}]; cnt]
PROG
(PARI) a(n) = my(N=10^n, s=0); forstep(k=9, N/2, [2, 2, 4, 2], s += !isprime(k) & !isprime(N-k)); s \\ Charles R Greathouse IV, May 17 2011
(PARI) a(n)=my(N=10^n, s=0, p=7); forprime(q=11, N/2, forstep(k=p+2, q-2, 2, s+=k%5&!isprime(N-k)); p=q); s+sum(k=precprime(N/2)+2, N/2, gcd(k, 10)==1&!isprime(N-k)) \\ Charles R Greathouse IV, May 17 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, May 17 2011
EXTENSIONS
a(9)-a(10) from Charles R Greathouse IV, May 18 2011
a(11) from Charles R Greathouse IV, May 20 2011
STATUS
approved