OFFSET
0,1
COMMENTS
A098147 is difference between first odd semiprime > 10^n and 10^n. How can we prove that there exists an a(n) for all n? In this powers of 3 sequence, does 2 occur infinitely often? Does every even integer k > 0 occur?
FORMULA
EXAMPLE
a(0) = 8 because 3^0 + 8 = 9 = 3^2 is an odd semiprime.
a(1) = 6 because 3^1 + 6 = 9 = 3^2 is an odd semiprime.
a(2) = 6 because 3^2 + 6 = 15 = 3 * 5 is an odd semiprime.
a(3) = 6 because 3^3 + 6 = 33 = 3 * 11 is an odd semiprime.
a(4) = 4 because 3^4 + 4 = 85 = 5 * 17 is an odd semiprime.
a(5) = 4 because 3^5 + 4 = 247 = 13 * 19 is an odd semiprime.
MATHEMATICA
a[n_] := Block[{z}, z = 3^n + 2; While[ PrimeOmega[z] != 2, z += 2]; z - 3^n]; a /@ Range[0, 60] (* Giovanni Resta, Jun 14 2016 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Feb 03 2006
EXTENSIONS
a(26) corrected and more terms from Giovanni Resta, Jun 14 2016
STATUS
approved