This one has been sticking out like a sore thumb for a while. I couldn’t figure it out for a long while until I found an algorithm for solving Pell numbers. I also used my old favourite prime number sieve. So I can’t really explain why this works. It just does. Better not question it and go on the next one.

#Euler66
from math import sqrt
class Algorithms:
def pell(self, d):
p, k, x1, y, sd = 1, 1, 1, 0, sqrt(d)
while k != 1 or y == 0:
p = k * (p/k+1) - p
p = p - int((p - sd)/k) * k
x = (p*x1 + d*y) / abs(k)
y = (p*y + x1) / abs(k)
k = (p*p - d) / k
x1 = x
return x
def primeGen (self, lowerLimit, upperLimit):
isPrime = [False, True]
i = 2
while i < upperLimit:
isPrime.append(True)
i += 1
p = 2
while p * p < upperLimit:
if isPrime[p] == True:
m = p * p
while m < upperLimit:
if m % p == 0:
isPrime[m] = False
m += p
p += 1
P = []
count = 0
for p in isPrime:
if p and count >= lowerLimit :
P.append(count)
count += 1
return P
class Problem66:
def __init__(self, limit):
self.limit = limit
self.Euler = Algorithms()
def Solution(self):
P = []
for p in self.Euler.primeGen(2,self.limit):
Pell = self.Euler.pell(p)
print Pell, p
P.append((Pell, p))
return "Answer = "+str(max(P)[1])
if __name__ == '__main__':
P = Problem66(1000)
print P.Solution()

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