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Subdomains work as well as sub folders within a website. The only reason not to use subdomains is that it may be easier to maintain a single website, than subdomains that act like separate websites. - Subdomains vs. Subfolders in SEO

• Matt Cutts suggesting to serve mobile content on a separate subdomain, and serving different content to mobile users and desktop users is fine and will not trigger duplicate content penalties - https://m.youtube.com/watch?v=mY9h3G8Lv4k

Link manipulation and spam and how they could affect domain authority - https://moz.com/blog/domain-authority-and-spam-detection

50 reasons your website may deserve a penalty from google: https://neilpatel.com/blog/penalized-by-google/

To read:

• Increasing organic SEO traffic 2013 - https://moz.com/blog/increasing-organic-seo-traffic-by-400000-unique-visitors-a-month