Primes with digits in nondecreasing order
Simple solution:
say 1000.primes.grep { .digits.cons(2).all { .head >= .tail } }
Generate such primes from digits (asymptotically faster):
func primes_with_nondecreasing_digits(upto, base = 10) {
upto = prev_prime(upto+1)
var list = []
var digits = @(1..^base -> flip)
var end_digits = digits.grep { .is_coprime(base) }
list << digits.grep { .is_prime && !.is_coprime(base) }...
for k in (0 .. upto.ilog(base)) {
digits.combinations_with_repetition(k, {|*a|
var v = a.digits2num(base)
next if (v*base + end_digits.tail > upto)
end_digits.each {|d|
var n = (v*base + d)
next if ((n >= base) && (a[0] > d))
list << n if n.is_prime
}
})
}
list.sort
}
say primes_with_nondecreasing_digits(1000)
Output:
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 37, 47, 59, 67, 79, 89, 113, 127, 137, 139, 149, 157, 167, 179, 199, 223, 227, 229, 233, 239, 257, 269, 277, 337, 347, 349, 359, 367, 379, 389, 449, 457, 467, 479, 499, 557, 569, 577, 599, 677]
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