OFFSET
1,1
EXAMPLE
a(1) = 5 because semiprime(5) = 14, whose sum of digits is 5, the same as its index as a semiprime.
MATHEMATICA
a = {}; c = 0; For[n = 4, n < 10000, n++, If[Sum[FactorInteger[n][[i, 2]], {i, 1, Length[FactorInteger[n]]}] == 2, c++; If[Plus @@ IntegerDigits[c] == Plus @@ IntegerDigits[n], AppendTo[a, c]]]]; a (* Stefan Steinerberger, Dec 29 2007 *)
SemiPrimePi[n_] := Sum[ PrimePi[n/Prime@i] - i + 1, {i, PrimePi@ Sqrt@n}]; SemiPrime[n_] := Block[{e = Floor[Log[2, n] + 1], a, b}, a = 2^e; Do[b = 2^p; While[SemiPrimePi@a < n, a = a + b]; a = a - b/2, {p, e, 0, -1}]; a + b/2]; Select[Range@ 1000, fQ@# &] (* Robert G. Wilson v *)
nn=5000; With[{sp=Select[Range[nn], PrimeOmega[#]==2&]}, Select[Range[ Length[sp]], Total[ IntegerDigits[sp[[#]]]] ==Total[ IntegerDigits[#]]&]] (* Harvey P. Dale, Oct 15 2012 *)
CROSSREFS
KEYWORD
base,easy,nonn,less
AUTHOR
Jonathan Vos Post, Dec 27 2007
EXTENSIONS
Corrected and extended by Stefan Steinerberger and Robert G. Wilson v, Dec 29 2007
STATUS
approved