🥖 add some erc20 info

Smart-Contract-Security/erc20-things.md

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-## ERC20 Things
-
-<br>
-
-### [What the ERC!?](https://medium.com/@adrianhetman/what-the-erc-d31bc3b3470a)
-
-* ERC stands for Ethereum request for comments. 
-* These are technical documents written by Ethereum developers for Ethereum community. 
-* Each such document contains a set of rules required to implement tokens for the Ethereum ecosystem.
-* You can say ERC is a specific type of EIP.
-
-
-<br>

Smart-Contract-Security/token-standards/erc20.md

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+## ERC20 
+
+<br>
+
+* The ERC20 standard defines a common interface for contracts implementing this token, such that any compatible token can be accessed and used in the same way. 
+
+<br>
+
+### ERC20-compliant token contract
+
+* totalSupply
+Returns the total units of this token that currently exist. ERC20 tokens can have a fixed or a variable supply.
+
+* balanceOf
+Given an address, returns the token balance of that address.
+
+* transfer
+Given an address and amount, transfers that amount of tokens to that address, from the balance of the address that executed the transfer.
+
+* transferFrom
+Given a sender, recipient, and amount, transfers tokens from one account to another. Used in combination with approve.
+
+* approve
+Given a recipient address and amount, authorizes that address to execute several transfers up to that amount, from the account that issued the approval.
+
+* allowance
+Given an owner address and a spender address, returns the remaining amount that the spender is approved to withdraw from the owner.
+
+* Transfer
+Event triggered upon a successful transfer (call to transfer or transferFrom) (even for zero-value transfers).
+
+* Approval
+Event logged upon a successful call to approve.
+
+<br>
+
+---
+
+### ERC20 optional functions
+
+In addition to the required functions listed in the previous section, the following optional functions are also defined by the standard:
+
+* name
+Returns the human-readable name (e.g., "US Dollars") of the token.
+
+* symbol
+Returns a human-readable symbol (e.g., "USD") for the token.
+
+* decimals
+Returns the number of decimals used to divide token amounts. For example, if decimals is 2, then the token amount is divided by 100 to get its user representation.
+
+<br>
+
+---
+
+### The ERC20 interface defined in Solidity
+
+
+```
+contract ERC20 {
+   function totalSupply() constant returns (uint theTotalSupply);
+   function balanceOf(address _owner) constant returns (uint balance);
+   function transfer(address _to, uint _value) returns (bool success);
+   function transferFrom(address _from, address _to, uint _value) returns
+      (bool success);
+   function approve(address _spender, uint _value) returns (bool success);
+   function allowance(address _owner, address _spender) constant returns
+      (uint remaining);
+   event Transfer(address indexed _from, address indexed _to, uint _value);
+   event Approval(address indexed _owner, address indexed _spender, uint _value);
+}
+```
+