def count_circular_primes(ceiling):
"""
Counts the number of circular primes below ceiling.
A circular prime is a prime for which all rotations of the digits is also
prime.
"""
return len([
a for a in range(ceiling)
if is_prime(a) and all(is_prime(a) for a in gen_rotation_list(a))
])
def truncatable(num):
"""
Returns true if num is a truncatable prime from left to right and right to
left.
A truncatable prime is a prime which remains prime as digits are removed
from either end.
"""
if len(str(num)) < 2 or not is_prime(num):
return False
return (all(is_prime(int(str(num)[:b])) for b in range(1, len(str(num))))
and all(
is_prime(int(str(num)[b:])) for b in range(1, len(str(num)))))
def spiral_ratio(threshold):
"""
Returns the side length of the square spiral for which the ratio of primes
along both diagonals first falls below threshold.
"""
prime_counter = 0
for sp in count(2):
prime_counter += sum(is_prime(a) for a in square_spiral_corners(sp))
if (prime_counter / (4 * sp - 3)) < threshold:
return 2 * sp - 1
def largest_pandigital_prime():
"""
Returns the largest n-digital pandigital prime.
"""
largest = 0
for setlen in range(9, 0, -1):
if largest > 0:
break
for c in generate(1, setlen):
if c > largest and is_prime(c):
largest = c
return largest
def quadratic_primes(bound):
"""
Returns the product of the coefficients a, b such that n^2 + an + b
produces the most consecutive primes for values of n starting at 0 where
abs(a) < bound and abs(b) <= bound.
"""
product_primes = [1, 0]
for a in range(-1 * bound + 1, bound):
for b in range(-1 * bound, bound + 1):
for n in count(0):
if not is_prime(n**2 + a * n + b):
break
if n > product_primes[1]:
product_primes = [a * b, n]
return product_primes[0]
def goldbach(vol=0):
"""
Finds the smallest odd composite that is not the sum of a prime and twice
a square.
"""
pset = set([2, 3, 5, 7])
for cand in count(9, 2):
if is_prime(cand):
pset.add(cand)
continue
cset = set(cand - 2 * x * x for x in range(1, int(sqrt(cand / 2)) + 1))
if cset & pset == set():
if vol >= 1:
print(f"{cand} is a counterexample.")
return cand
def smallestprimemember(family, searchlen):
"""
Returns the smallest prime of the family if the family has size at least
searchlen. Otherwise returns 0.
"""
primes = []
for d in range(10):
if d == 0 and family[0] == "*":
# Cannot replace leading star with zero
continue
if 10 - d < searchlen - len(primes):
# Not enough left to satisfy searchlen
return 0
num = replacestar(family, d)
if (is_prime(num)):
primes.append(num)
if len(primes) >= searchlen:
return primes[0]
else:
return 0
def pairwise(a, b):
"""
Determines if the concatenation of a and b in both ways is prime
"""
return is_prime(int(str(b) + str(a))) and is_prime(int(str(a) + str(b)))