The Wikipedia article on the sieve of Eratosthenes mentions a segmented approach, which is also described in the Sorenson paper in section 5. The main insight is that we only need the primes up to to be able to sieve a table all the way to N. This results in a sieve that uses only memory. Here's a commented Python implementation:

``````import math

def gen_primes_upto(n):
"""Generates a sequence of primes < n.

Uses the full sieve of Eratosthenes with O(n) memory.
"""
if n == 2:
return

# Initialize table; True means "prime", initially assuming all numbers
# are prime.
table = [True] * n
sqrtn = int(math.ceil(math.sqrt(n)))

# Starting with 2, for each True (prime) number I in the table, mark all
# its multiples as composite (starting with I*I, since earlier multiples
# should have already been marked as multiples of smaller primes).
# At the end of this process, the remaining True items in the table are
# primes, and the False items are composites.
for i in range(2, sqrtn):
if table[i]:
for j in range(i * i, n, i):
table[j] = False

# Yield all the primes in the table.
yield 2
for i in range(3, n, 2):
if table[i]:
yield i

def gen_primes_upto_segmented(n):
"""Generates a sequence of primes < n.

Uses the segmented sieve or Eratosthenes algorithm with O(√n) memory.
"""
# Simplify boundary cases by hard-coding some small primes.
if n < 11:
for p in [2, 3, 5, 7]:
if p < n:
yield p
return

# We break the range [0..n) into segments of size √n
segsize = int(math.ceil(math.sqrt(n)))

# Find the primes in the first segment by calling the basic sieve on that
# segment (its memory usage will be O(√n)). We'll use these primes to
# sieve all subsequent segments.
baseprimes = list(gen_primes_upto(segsize))
for bp in baseprimes:
yield bp

for segstart in range(segsize, n, segsize):
# Create a new table of size √n for each segment; the old table
# is thrown away, so the total memory use here is √n
# seg[i] represents the number segstart+i
seg = [True] * segsize

for bp in baseprimes:
# The first multiple of bp in this segment can be calculated using
# modulo.
first_multiple = (
segstart if segstart % bp == 0 else segstart + bp - segstart % bp
)
# Mark all multiples of bp in the segment as composite.
for q in range(first_multiple, segstart + segsize, bp):
seg[q % len(seg)] = False

# Sieving is done; yield all composites in the segment (iterating only
# over the odd ones).
start = 1 if segstart % 2 == 0 else 0
for i in range(start, len(seg), 2):
if seg[i]:
if segstart + i >= n:
break
yield segstart + i

# Usage:
for i in gen_primes_upto_segmented(1000):
print(i)
``````