Friedman number
Encyclopedia
A Friedman number is an integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...

which, in a given base, is the result of an expression using all its own digits in combination with any of the four basic arithmetic operators (+, −, ×, ÷) and sometimes exponentiation
Exponentiation
Exponentiation is a mathematical operation, written as an, involving two numbers, the base a and the exponent n...

. For example, 347 is a Friedman number since 347 = 73 + 4. The first few base 10 Friedman numbers are:
25, 121, 125, 126, 127, 128, 153, 216, 289, 343, 347, 625, 688, 736, 1022, 1024, 1206, 1255, 1260, 1285, 1296, 1395, 1435, 1503, 1530, 1792, 1827, 2048, 2187, 2349, 2500, 2501, 2502, 2503, 2504, 2505, 2506, 2507, 2508, 2509, 2592, 2737, 2916, 3125, 3159 .

Parentheses can be used in the expressions, but only to override the default operator precedence, for example, in 1024 = (4 − 2)10. Allowing parentheses without operators would result in trivial Friedman numbers such as 24 = (24). Leading zeros cannot be used, since that would also result in trivial Friedman numbers, such as 001729 = 1700 + 29.

A nice Friedman number is a Friedman number where the digits in the expression can be arranged to be in the same order as in the number itself. For example, we can arrange 127 = 27 − 1 as 127 = −1 + 27. The first nice Friedman numbers are:
127, 343, 736, 1285, 2187, 2502, 2592, 2737, 3125, 3685, 3864, 3972, 4096, 6455, 11264, 11664, 12850, 13825, 14641, 15552, 15585, 15612, 15613, 15617, 15618, 15621, 15622, 15623, 15624, 15626, 15632, 15633, 15642, 15645, 15655, 15656, 15662, 15667, 15688, 16377, 16384, 16447, 16875, 17536, 18432, 19453, 19683, 19739 .

Currently, 81 zeroless pandigital
Pandigital number
In mathematics, a pandigital number is an integer that in a given base has among its significant digits each digit used in the base at least once. For example, 1223334444555567890 is a pandigital number in base 10...

Friedman numbers are known. Two of them are: 123456789 = ((86 + 2 × 7)5 − 91) / 34, and 987654321 = (8 × (97 + 6/2)5 + 1) / 34, both discovered by Mike Reid and Philippe Fondanaiche. Only one of the 81 known zeroless pandigital Friedman numbers is nice: 268435179 = −268 + 4(3×5 − 17) − 9.

From the observation that all numbers of the form 25×102n can be written as 500...02 with n 0's, we can find strings of consecutive Friedman numbers. Friedman gives the example of 250068 = 5002 + 68, from which we can easily deduce the range of consecutive Friedman numbers from 250000 to 250099.

It seems that all powers of 5 are Friedman numbers.

Fondanaiche thinks the smallest repdigit
Repdigit
In recreational mathematics, a repdigit is a natural number composed of repeated instances of the same digit, most often in the decimal numeral system....

nice Friedman number is 99999999 = (9 + 9/9)9−9/9 − 9/9. Brandon Owens proved that repdigits of more than 24 digits are nice Friedman numbers in any base.

Vampire number
Vampire number
In mathematics, a vampire number is a composite natural number v, with an even number of digits n, that can be factored into two integers x and y each with n/2 digits and not both with trailing zeroes, where v contains precisely all the digits from x and from y, in any order, counting multiplicity...

s are a type of Friedman numbers where the only operation is a multiplication of two numbers with the same number of digits, for example 1260 = 21 × 60.

## Finding 2-digit Friedman numbers

There usually are fewer 2-digit Friedman numbers than 3-digit and more in any given base, but the 2-digit ones are easier to find. If we represent a 2-digit number as mb + n, where b is the base and m, n are integers from 0 to b−1, we need only check each possible combination of m and n against the equalities mb + n = mn, and mb + n = nm to see which ones are true. We need not concern ourselves with m + n or m × n, since these will always be smaller than mb + n when n < b. The same clearly holds for mn and m/n.

## Friedman numbers using Roman numerals

In a trivial sense, all Roman numerals with more than one symbol are Friedman numbers. The expression is created by simply inserting + signs into the numeral, and occasionally the − sign with slight rearrangement of the order of the symbols.

But Erich Friedman and Robert Happelberg have done some research into Roman numeral Friedman numbers for which the expression uses some of the other operators. Their first discovery was the nice Friedman number 8, since VIII = (V - I) × II. They have also found many Roman numeral Friedman numbers for which the expression uses exponentiation, e.g., 256 since CCLVI = IVCC/L.

The difficulty of finding nontrivial Friedman numbers in Roman numerals increases not with the size of the number (as is the case with positional notation
Positional notation
Positional notation or place-value notation is a method of representing or encoding numbers. Positional notation is distinguished from other notations for its use of the same symbol for the different orders of magnitude...

numbering systems) but with the numbers of symbols it has. So, for example, it is much tougher to figure out whether 147 (CXLVII) is a Friedman number in Roman numerals than it is to make the same determination for 1001 (MI). With Roman numerals, one can at least derive quite a few Friedman expressions from any new expression one discovers. Friedman and Happelberg have shown that any number ending in VIII is a Friedman number based on the expression given above, for instance.