MiSawa

#!/usr/bin/ruby
def f(n)
  n.to_s.split('').map(&:to_i).inject(0){|a,b|a+b**2}
end
a = Hash.new do |hash, n|
  hash[n] = (n == 1 || n == 89) ? n : hash[f(n)]
end
p 1.upto(1E7).count{|n|a[f(n)] == 89}
#=> 8581146

MiSawa

p 1.upto(9999).count{|n|
  50.times.all?{
    n += n.to_s.reverse.to_i
    n.to_s != n.to_s.reverse
  }
}
 #=> 249

MiSawa

$facto = [1]
for i in 1..10
  $facto[i] = $facto[i-1] * i
end
def f(n)
  res = 0
  until n.zero?
    res += $facto[n%10]
    n /= 10
  end

MiSawa

require "prime"

mx = 1000
res = [0, 0, 0]
for b in -mx+1...mx
  next unless b.prime?
  for a in -mx+1...mx
    l = 0
    l += 1 while (l*l + a*l + b).prime?
    res = [res, [l, a, b]].max

MiSawa

require "mathn"
def f(x)
  a = []
  hash = {}
  n = 1/x
  while hash[n].nil?
    hash[n] = true
    n *= 10
    n -= n.to_i
  end