Golf:

  strong :: (Integral i) => [i] -> [i]
  strong λ@(_:b:c) = view _2 <$> filter p (zip3 λ (b:c) c)
    where p (x, y, z) = 2 * y > x + z

Also, it hangs with scanr (scanl works) and includes 2 which isn't strong.