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487 Sums of power sums.pl
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487 Sums of power sums.pl
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#!/usr/bin/perl
# Daniel "Trizen" Șuteu
# Date: 28 June 2019
# https://github.com/trizen
# https://projecteuler.net/problem=487
# Runtime: 44.486s
use 5.020;
use strict;
use warnings;
use experimental qw(signatures);
use Math::GMPz;
use Math::GMPq;
use ntheory qw(forprimes addmod submod divmod mulmod powmod);
sub tangent_numbers ($n) {
my @T = (Math::GMPz::Rmpz_init_set_ui(1));
foreach my $k (1 .. $n) {
Math::GMPz::Rmpz_mul_ui($T[$k] = Math::GMPz::Rmpz_init(), $T[$k - 1], $k);
}
foreach my $k (1 .. $n) {
foreach my $j ($k .. $n) {
Math::GMPz::Rmpz_mul_ui($T[$j], $T[$j], $j - $k + 2);
Math::GMPz::Rmpz_addmul_ui($T[$j], $T[$j - 1], $j - $k);
}
}
return @T;
}
sub bernoulli_numbers ($n) {
$n = ($n >> 1) + 1;
my @B;
my @T = tangent_numbers($n);
my $t = Math::GMPz::Rmpz_init();
foreach my $k (0 .. 2 * @T) {
$k % 2 == 0 or $k == 1 or next;
my $q = Math::GMPq::Rmpq_init();
if ($k == 0) {
Math::GMPq::Rmpq_set_ui($q, 1, 1);
$B[$k] = $q;
next;
}
if ($k == 1) {
Math::GMPq::Rmpq_set_si($q, -1, 2);
$B[$k] = $q;
next;
}
# T_k
Math::GMPz::Rmpz_mul_ui($t, $T[($k >> 1) - 1], $k);
Math::GMPz::Rmpz_neg($t, $t) if ((($k >> 1) - 1) & 1);
Math::GMPq::Rmpq_set_z($q, $t);
# (2^k - 1) * 2^k
Math::GMPz::Rmpz_set_ui($t, 0);
Math::GMPz::Rmpz_setbit($t, $k);
Math::GMPz::Rmpz_sub_ui($t, $t, 1);
Math::GMPz::Rmpz_mul_2exp($t, $t, $k);
# B_k = q
Math::GMPq::Rmpq_div_z($q, $q, $t);
$B[($k >> 1) + 1] = $q;
}
return @B;
}
say ":: Computing Bernoulli numbers...";
my $power = 1e4;
my @bernoulli = map {
my $num = Math::GMPz->new(0);
my $den = Math::GMPz->new(0);
Math::GMPq::Rmpq_get_num($num, $_);
Math::GMPq::Rmpq_get_den($den, $_);
[$num, $den];
} bernoulli_numbers($power + 1);
say ":: Applying Faulhaber's formula...";
my $B0 = [map { Math::GMPz->new($_) } (1, 1)];
my $B1 = [map { Math::GMPz->new($_) } (1, 2)];
{
my @cache;
my $t = Math::GMPz::Rmpz_init_nobless();
sub binomialmod ($n, $k, $m) {
my $bin = (
$cache[$n][$k] //= do {
my $z = Math::GMPz::Rmpz_init_nobless();
Math::GMPz::Rmpz_bin_uiui($z, $n, $k);
$z;
}
);
Math::GMPz::Rmpz_mod_ui($t, $bin, $m);
}
}
# Faulhaber's formula (modulo some m)
# See: https://en.wikipedia.org/wiki/Faulhaber%27s_formula
sub faulhabermod ($n, $p, $m) {
my $sum = 0;
for my $k (0 .. $p) {
$k % 2 == 0 or $k == 1 or next;
my $B =
($k == 0) ? $B0
: ($k == 1) ? $B1
: $bernoulli[($k >> 1) + 1];
$sum += mulmod(divmod(mulmod(binomialmod($p + 1, $k, $m), $B->[0] % $m, $m), $B->[1] % $m, $m),
powmod($n, $p - $k + 1, $m), $m);
$sum %= $m;
}
divmod($sum, $p + 1, $m);
}
# Efficient formula for computing:
# S_p(n) = Sum_{k=1..n} Sum_{j=1..k} j^p
# for some positive integer p.
# The formula is:
# S_p(n) = (n+1) * F_p(n) - F_(p+1)(n)
# where F_n(x) are the Faulhaber polynomials.
sub S ($n, $p, $mod) {
submod(mulmod($n + 1, faulhabermod($n, $p, $mod), $mod), faulhabermod($n, $p + 1, $mod), $mod);
}
# Sanity check
if (S(100, 4, 1e9 + 1) != (35375333830 % (1e9 + 1))) {
die "Error for S_4(100)";
}
my $sum = 0;
forprimes { # there are only 100 primes in this range
$sum += S(1e12, $power, $_);
} 2e9, 2e9 + 2000;
say ":: Total: $sum";