Number Dictionary - Divisors+

Number Dictionary lets you pick 2 to 4 numbers and instantly see every relationship that holds between them — amicable numbers, twin primes, arithmetic sequences, and 16 more. It also includes a 296-entry dictionary of number properties you can look up one number at a time, plus answer-checking for divisors, multiples, and prime factorization homework. The app has four tabs: Compare, Search, Reverse Lookup, and School. Compare takes 2 to 4 numbers and lists every relationship that applies — GCD, LCM, amicable numbers, twin primes, arithmetic/geometric sequences, Pythagorean triples, and more. Search takes one number and ranks its properties from rare to common. Reverse Lookup starts from a property's name and finds numbers that match it. School narrows the list to a specific level — elementary, middle school, high school, or beyond school — organized by unit. ■ What Compare can tell you (new, September 2026) • Greatest common divisor / least common multiple / whether two numbers are coprime • Amicable numbers and sociable numbers (chains of numbers linked by their divisor sums) • Twin primes, cousin primes, sexy primes, and prime quadruplets • Arithmetic sequences, geometric sequences, and consecutive Fibonacci numbers • Pythagorean triples and quadruples • Properties of the sum or product of the numbers you selected ■ What you can learn • Answer-checking for divisors, multiples, GCD, LCM, and prime factorization, with the working shown • A 296-entry dictionary of number properties — prime number facts, divisor relationships like perfect and amicable numbers, figurate numbers, named numbers, sequences like the Collatz conjecture, notations like different number bases and Roman numerals, constants like pi, PIN strength, and properties organized by school unit • Reverse lookup by property (for example, a full list of Harshad numbers) • Diagrams for some properties, such as a prime factorization tree • A daily feature that factors today's date and shows its properties • A School tab that organizes properties by level — elementary, middle school, high school, or beyond school ■ Designed to be easy to use • Results are shown as holds / can't tell / doesn't hold, so an unclear case never gets mistaken for a match • Rarity is shown as a star rating and a plain "1 in every N" figure • Every value comes with the formula behind it, so you can see why, not just what • Every entry in the School tab includes a worked example, not just a unit name • No account needed — everything you enter is processed on the spot and never leaves your device The app shows a banner ad at the bottom of the screen. A one-time in-app purchase removes ads. There's no reward-ad tier, no subscription, and no AI explanation feature — every one of the 296 dictionary entries is free, because a dictionary only works if nothing in it is locked away.