https://en.bitcoin.it/wiki/Irreversible_Transactions
https://www.reddit.com/r/Bitcoin/comments/2e7bfa/vector76_double_spend_attack/cjwya6x
http://36kr.com/p/5137942.html
https://coinsutra.com/bitcoin-double-spending/
https://bitcoil.co.il/Doublespend.pdf
https://en.bitcoin.it/wiki/Irreversible_Transactions#Attack_vectors
https://eprint.iacr.org/2012/248.pdf
https://99bitcoins.com/double-spending/
https://en.bitcoin.it/wiki/Confirmation
https://medium.com/@bramcohen/the-inevitable-demise-of-unconfirmed-bitcoin-transactions-8b5f66a44a35
https://en.bitcoin.it/wiki/Myths#Bitcoin_has_no_built-in_chargeback_mechanism_and_this_is_bad