# Zumkeller numbers

``````func is_Zumkeller(n) {

return false if n.is_prime
return false if n.is_square

var sigma = n.sigma

# n must have an even abundance
return false if (sigma.is_odd || (sigma < 2*n))

# true if n is odd and has an even abundance
return true if n.is_odd    # conjecture

var divisors = n.divisors

for k in (2 .. divisors.end) {
divisors.combinations(k, {|*a|
if (2*a.sum == sigma) {
return true
}
})
}

return false
}

say "First 220 Zumkeller numbers:"
say (1..Inf -> lazy.grep(is_Zumkeller).first(220).join(' '))

say "\nFirst 40 odd Zumkeller numbers: "
say (1..Inf `by` 2 -> lazy.grep(is_Zumkeller).first(40).join(' '))

say "\nFirst 40 odd Zumkeller numbers not divisible by 5: "
say (1..Inf `by` 2 -> lazy.grep { _ % 5 != 0 }.grep(is_Zumkeller).first(40).join(' '))
``````

#### Output:

``````First 220 Zumkeller numbers:
6 12 20 24 28 30 40 42 48 54 56 60 66 70 78 80 84 88 90 96 102 104 108 112 114 120 126 132 138 140 150 156 160 168 174 176 180 186 192 198 204 208 210 216 220 222 224 228 234 240 246 252 258 260 264 270 272 276 280 282 294 300 304 306 308 312 318 320 330 336 340 342 348 350 352 354 360 364 366 368 372 378 380 384 390 396 402 408 414 416 420 426 432 438 440 444 448 456 460 462 464 468 474 476 480 486 490 492 496 498 500 504 510 516 520 522 528 532 534 540 544 546 550 552 558 560 564 570 572 580 582 588 594 600 606 608 612 616 618 620 624 630 636 640 642 644 650 654 660 666 672 678 680 684 690 696 700 702 704 708 714 720 726 728 732 736 740 744 750 756 760 762 768 770 780 786 792 798 804 810 812 816 820 822 828 832 834 836 840 852 858 860 864 868 870 876 880 888 894 896 906 910 912 918 920 924 928 930 936 940 942 945 948 952 960 966 972 978 980 984

First 40 odd Zumkeller numbers:
945 1575 2205 2835 3465 4095 4725 5355 5775 5985 6435 6615 6825 7245 7425 7875 8085 8415 8505 8925 9135 9555 9765 10395 11655 12285 12705 12915 13545 14175 14805 15015 15435 16065 16695 17325 17955 18585 19215 19305

First 40 odd Zumkeller numbers not divisible by 5:
81081 153153 171171 189189 207207 223839 243243 261261 279279 297297 351351 459459 513513 567567 621621 671517 729729 742203 783783 793611 812889 837837 891891 908523 960687 999999 1024947 1054053 1072071 1073709 1095633 1108107 1145529 1162161 1198197 1224531 1270269 1307691 1324323 1378377
``````