def solve():
def p(i):
return i * (3 * i - 1) / 2
j = 1
while True:
j += 1
pj = p(j)
for k in reversed(range(1, j)):
pk = p(k)
if is_pentagonal(pj - pk) and is_pentagonal(pj + pk):
return int(pj - pk)
def solve():
# t(n) = n(n + 1) / 2 für n = 2n - 1
# = (2n - 1)(2n - 1 + 1) / 2
# = (2n - 1)(2n) / 2
# = n(2n - 1)
# = h(n)
#
# Therefore t(n) is a subset of h(n)
def h(n):
return n * (2 * n - 1)
# Start from 144 as the answer must be greater than h(143)
i = 144
while True:
formula = h(i)
if is_pentagonal(formula):
return formula
i += 1