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IDDescription / TitleSolved By
401Sum of squares of divisors
402Integer-valued polynomials
403Lattice points enclosed by parabola and line
404Crisscross Ellipses
405A rectangular tiling
406Guessing Game
408Admissible paths through a grid
409Nim Extreme
410Circle and tangent line
411Uphill paths
412Gnomon numbering
413One-child Numbers
414Kaprekar constant
415Titanic sets
416A frog's trip
417Reciprocal cycles II
418Factorisation triples
419Look and say sequence
4202x2 positive integer matrix
421Prime factors of n15+1
422Sequence of points on a hyperbola
423Consecutive die throws
425Prime connection
426Box-ball system
428Necklace of circles
429Sum of squares of unitary divisors
430Range flips
431Square Space Silo
432Totient sum
433Steps in Euclid's algorithm
434Rigid graphs
435Polynomials of Fibonacci numbers
436Unfair wager
437Fibonacci primitive roots
438Integer part of polynomial equation's solutions
439Sum of sum of divisors
440GCD and Tiling
441The inverse summation of coprime couples
442Eleven-free integers
443GCD sequence
444The Roundtable Lottery
445Retractions A
446Retractions B
447Retractions C
448Average least common multiple
449Chocolate covered candy
450Hypocycloid and Lattice points