This pattern is available in the Winter 2015 issue of Cast-On Magazine, and will be available for individual pdf download in the Winter of 2016.
The sleeves and body are knit separately form the bottom up to the underarms where they are joined. Decreases are worked on the fronts and back, then the sleeve caps are decreased. The front neck shaping begins before the sleeve cap shaping is completed. A few short rows are worked to finish off the tops of the sleeve caps, then the back and front stitches are joined with a three-needle bind-off for the shoulders. The underarm stitches are returned to double point needles and are joined with a three-needle bind-off. The button bands and collar stitches are picked up and knit. This is a completely seamless sweater.