IntegerString

IntegerString[n] gives a string consisting of the decimal digits in the integer n. 

IntegerString[n,b] gives a string consisting of the base-b digits in the integer n. 

IntegerString[n,b,len] pads the string on the left with zero digits to give a string of length len. 

IntegerString[n,MixedRadix[blist]] uses the mixed radix with a list of bases blist.

IntegerString[n,"numsys"] gives the numeral form of n based on the numeric system defined by "numsys".

Please visit the official Wolfram Language Reference for more details and examples on core symbols.