Freshman's dream

• ${\displaystyle (1+4)^{2}=5^{2}=25}$ , but ${\displaystyle 1^{2}+4^{2}=17}$ .
• ${\displaystyle {\sqrt {x^{2}+y^{2}}}}$  does not equal ${\displaystyle {\sqrt {x^{2}}}+{\sqrt {y^{2}}}=|x|+|y|}$ . For example, ${\displaystyle {\sqrt {9+16}}={\sqrt {25}}=5}$ , which does not equal 3 + 4 = 7. In this example, the error is being committed with the exponent n = ⁠1/2⁠.

When ${\displaystyle p}$  is a prime number and ${\displaystyle x}$  and ${\displaystyle y}$  are members of a commutative ring of characteristic ${\displaystyle p}$ , then ${\displaystyle (x+y)^{p}=x^{p}+y^{p}}$ . This can be seen by examining the prime factors of the binomial coefficients: the nth binomial coefficient is

${\displaystyle {\binom {p}{n}}={\frac {p!}{n!(p-n)!}}.}$ 

The numerator is p factorial(!), which is divisible by p. However, when 0 < n < p, both n! and (p − n)! are coprime with p since all the factors are less than p and p is prime. Since a binomial coefficient is always an integer, the nth binomial coefficient is divisible by p and hence equal to 0 in the ring. We are left with the zeroth and pth coefficients, which both equal 1, yielding the desired equation.

Thus in characteristic p the freshman's dream is a valid identity. This result demonstrates that exponentiation by p produces an endomorphism, known as the Frobenius endomorphism of the ring.

The demand that the characteristic p be a prime number is central to the truth of the freshman's dream. A related theorem states that if p is prime then (x + 1)^p ≡ x^p + 1 in the polynomial ring ${\displaystyle \mathbb {Z} _{p}[x]}$ . This theorem is a key fact in modern primality testing.^[4]

In 1938, Harold Willard Gleason published a poem titled «"Dark and Bloody Ground---" (The Freshman's Dream)» in The New York Sun on September 6, which was subsequently reprinted in various other newspapers and magazines. It consists of 2 stanzas, each containing 8 lines with alternating indentation; it has an ABCB rhyming scheme. Words and phrases that hint that it might be related to this concept include: "Algebra", "Wild corollaries twine", "surds", "of plus and minus sign", "binomial", "quadratic", "parenthesis", "exponents", "in terms of x and y", "remove the brackets, radicals, and do so with discretion", and "factor cubes".^[5]

The history of the term "freshman's dream" is somewhat unclear. In a 1940 article on modular fields, Saunders Mac Lane quotes Stephen Kleene's remark that a knowledge of (a + b)² = a² + b² in a field of characteristic 2 would corrupt freshman students of algebra. This may be the first connection between "freshman" and binomial expansion in fields of positive characteristic.^[6] Since then, authors of undergraduate algebra texts took note of the common error. The first actual attestation of the phrase "freshman's dream" seems to be in Hungerford's graduate algebra textbook (1974), where he states that the name is "due to" Vincent O. McBrien.^[7] Alternative terms include "freshman exponentiation", used in Fraleigh (1998).^[8] The term "freshman's dream" itself, in non-mathematical contexts, is recorded since the 19th century.^[9]

Since the expansion of (x + y)ⁿ is correctly given by the binomial theorem, the freshman's dream is also known as the "child's binomial theorem"^[4] or "schoolboy binomial theorem".

• Pons asinorum
• Primality test
• Sophomore's dream
• Frobenius endomorphism