sidef-scripts/Math/carmichael_numbers_in_range.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 29 August 2022
# https://github.com/trizen

# Generate all the Carmichael numbers with n prime factors in a given range [a,b]. (not in sorted order)

# See also:
#   https://en.wikipedia.org/wiki/Almost_prime
#   https://trizenx.blogspot.com/2020/08/pseudoprimes-construction-methods-and.html

#`{

# PARI/GP program (slow):

carmichael(A, B, k) = A=max(A, vecprod(primes(k+1))\2); (f(m, l, p, k, u=0, v=0) = my(list=List()); if(k==1, forprime(p=u, v, my(t=m*p); if((t-1)%l == 0 && (t-1)%(p-1) == 0, listput(list, t))), forprime(q = p, sqrtnint(B\m, k), my(t = m*q); my(L=lcm(l, q-1)); if(gcd(L, t) == 1, my(u=ceil(A/t), v=B\t); if(u <= v, my(r=nextprime(q+1)); if(k==2 && r>u, u=r); list=concat(list, f(t, L, r, k-1, u, v)))))); list); vecsort(Vec(f(1, 1, 3, k)));

# PARI/GP program (in range) (faster):

carmichael(A, B, k) = A=max(A, vecprod(primes(k+1))\2); my(max_p=(1+sqrtint(8*B+1))\4); (f(m, l, lo, k) = my(list=List()); my(hi=min(max_p, sqrtnint(B\m, k))); if(lo > hi, return(list)); if(k==1, lo=max(lo, ceil(A/m)); my(t=lift(1/Mod(m,l))); while(t < lo, t += l); forstep(p=t, hi, l, if(isprime(p), my(n=m*p); if((n-1)%(p-1) == 0, listput(list, n)))), forprime(p=lo, hi, if(gcd(m, p-1) == 1, list=concat(list, f(m*p, lcm(l, p-1), p+1, k-1))))); list); vecsort(Vec(f(1, 1, 3, k)));

# PARI/GP program to generate all the Carmichael numbers <= n (fast):

carmichael(A, B, k) = A=max(A, vecprod(primes(k+1))\2); my(max_p=(1+sqrtint(8*B+1))\4); (f(m, l, lo, k) = my(list=List()); my(hi=min(max_p, sqrtnint(B\m, k))); if(lo > hi, return(list)); if(k==1, lo=max(lo, ceil(A/m)); my(t=lift(1/Mod(m,l))); while(t < lo, t += l); forstep(p=t, hi, l, if(isprime(p), my(n=m*p); if((n-1)%(p-1) == 0, listput(list, n)))), forprime(p=lo, hi, if(gcd(m, p-1) == 1, list=concat(list, f(m*p, lcm(l, p-1), p+1, k-1))))); list); f(1, 1, 3, k);
upto(n) = my(list=List()); for(k=3, oo, if(vecprod(primes(k+1))\2 > n, break); list=concat(list, carmichael(1, n, k))); vecsort(Vec(list));

}

func carmichael_numbers_in_range(A, B, k, callback) {

    A = max(pn_primorial(k+1)/2, A)

    # Largest possisble prime factor for Carmichael numbers <= B
    var max_p = ((1 + isqrt(8*B + 1))>>2)

    func (m, L, lo, k) {

        var hi = idiv(B,m).iroot(k)

        if (lo > hi) {
            return nil
        }

        if (k == 1) {

            hi = max_p if (hi > max_p)
            lo = max(lo, idiv_ceil(A, m))
            lo > hi && return nil

            var t = m.invmod(L)

            t > hi && return nil
            t += L*idiv_ceil(lo - t, L) if (t < lo)
            t > hi && return nil

            for p in (range(t, hi, L)) {
                p.is_prime || next
                with (m*p) {|n|
                    if (p.dec `divides` n.dec) {
                        callback(n)
                    }
                }
            }

            return nil
        }

        each_prime(lo, hi, {|p|
            m.is_coprime(p-1) || next
            __FUNC__(m*p, lcm(L, p-1), p+1, k-1)
        })
    }(1, 1, 3, k)

    return callback
}

# High-level implementation (unoptimized):
func carmichael_numbers_in_range_unoptimized(A, B, k, callback) {

    func F(m, lambda, p, k) {
        if (k == 1) {
            each_prime(max(p, ceil(A/m)), min(floor((1+isqrt(8*B + 1))/4), floor(B/m)), {|q|
                if (lcm(lambda, q-1) `divides` (m*q - 1)) {
                    callback(m*q)
                }
            })
        }
        elsif (k >= 2) {
            for q in (primes(p, floor((B/m)**(1/k)))) {
                if (gcd(lcm(lambda, q-1), m) == 1) {
                    F(m*q, lcm(lambda, q-1), q+1, k-1)
                }
            }
        }
    }

    F(1, 1, 3, k)
}

# Generate all the 5-Carmichael numbers in the range [100, 2*10^7]

var k    = 5
var from = 100
var upto = 2e7

say gather { carmichael_numbers_in_range(from, upto, k, { take(_) }) }.sort
say gather { carmichael_numbers_in_range_unoptimized(from, upto, k, { take(_) }) }.sort

__END__
[825265, 1050985, 9890881, 10877581, 12945745, 13992265, 16778881, 18162001]