JSchlather
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 76 Can you answer my son's fourth-grade homework question: Which numbers are prime, have digits adding to ten and have a three in the tens place? 52 Paul Erdos's Two-Line Functional Analysis Proof 17 Compute $\operatorname{Hom}_{\mathbb{Z}} ( \mathbb{Q}, \mathbb{Z})$ 14 Non-associative, non-commutative binary operation with a identity 12 Why is the supremum of the empty set $-\infty$ and the infimum $\infty$?

### Reputation (11,118)

 +30 Can you answer my son's fourth-grade homework question: Which numbers are prime, have digits adding to ten and have a three in the tens place? +10 A good introduction to elementary algebra? +5 Why is $S/R$ a ring extension? +5 A Tricky Limit: $\lim \limits_{x\rightarrow 0} \frac{9^x-5^x}{x}$ without L'Hospital

### Questions (29)

 19 Bijection between $2^{\mathbb R}$ and $\mathbb{ R ^ R}$ 14 Why is the Long Line not a covering space for the Circle 12 Algebraic Proof that $\sum\limits_{i=0}^n \binom{n}{i}=2^n$ 11 Whats the probability a subset of an $\mathbb F_2$ vector space is a spanning set? 11 How is addition defined?

### Tags (101)

 245 abstract-algebra × 72 71 galois-theory × 25 147 field-theory × 53 71 reference-request × 10 112 general-topology × 40 69 group-theory × 18 76 elementary-number-theory 67 real-analysis × 21 76 recreational-mathematics 63 functional-analysis × 7

### Accounts (16)

 Mathematics 11,118 rep 32463 Stack Overflow 1,156 rep 819 TeX - LaTeX 356 rep 114 Area 51 151 rep 1 MathOverflow 123 rep 3