# Excellent Numbers

A number `n`, with an even number of digits, is excellent if it can be split into two halves, `a` and `b`, such that `b2 - a2 = n`. Let `2k` be the number of digits, then we want `n = aA + b = b2 - a2 where A = 10k`.

Let’s do some algebra:

```aA + b = b2 - a2 a2 + aA = b2 - b # Rearrange 4a2 + 4aA = 4b2 - 4b # Multiply by 4 (2a + A)2 - A2 = (2b - 1)2 - 1 # Complete the square ```

Now we can substitute `X = 2a + A`, `Y = 2b - 1`, `N = A2 - 1` and rearranging a little, we have:

```X2 - Y2 = N (X - Y)(X + Y) = N ```

So, every `2k` digit excellent number gives rise to divisors `i,j` of `N` where `ij = N` and `i <= j`

This process can be reversed: if `i` is a divisor of `N`, with `j = N/i` and `i <= j`, we have `X = (j+i)/2`, `Y = (j-i)/2`, then `a = (X-A)/2` and `b = (Y+1)/2`. If all the divisions by 2 are exact (and in this case they are – `N` is odd, so `i` and `j` are too, also writing `i = 2i'+1` and `j = 2j'+1`, we can show that `i'` and `j'` must have different parities) then we have a potentially excellent number – all we need to check is that `a` has exactly `k` digits and that `b` has at most `k` (otherwise `a` and `b` are not the upper and lower halves of a `2k` digit number).

Now we have a nice algorithm: find all divisors `i` of `N = 10k-1`, with `i <= sqrt(N)`, find `a` and `b` as above and check if they are in the appropriate range, if so, we have an excellent number and it should be clear that all excellent numbers can be generated in this way.

For small `N`, we can find all divisors just by a linear scan, but for larger `N` something better is needed: given a prime factorization we can generate all possible combinations of the factors to get the divisors, so now all we need to do is factorize `102k-1`. This of course is a hard problem but we can use, for example, Python’s `primefac` library, and give it some help by observing that `102k-1 = (10k-1)(10k+1)`. The factorization is harder for some values of `k`, particularly if `k` is prime, but we can always have a look at:

http://stdkmd.com/nrr/repunit/

if we run in to trouble. My Pi 2 gets stuck at `k = 71` where 11, 290249, 241573142393627673576957439049, 45994811347886846310221728895223034301839 and 31321069464181068355415209323405389541706979493156189716729115659 are the factors needed, so it’s not surprising it is struggling. Also, the number of divisors to check is approximately `2n-1` where `n` is the number of prime factors, of which, for example `1090-1` has 35 so just generating all potential 180 digit numbers will take a while.

So, after all that, here’s some code. Using Python generators keeps the memory usage down – we can process each divisor as it is constructed, (though it does mean that results for a particular size don’t come out in order) – after running for around 24 hours on a Pi 2, we are up to 180 digits and around 2000000 numbers but `top` reports less than 1% of memory in use.

```import primefac

def excellent(k):
"""Generate all excellent numbers of size 2k"""
A = 10**k; N = A*A-1
factors1 = list(primefac.primefac(A-1))
factors2 = list(primefac.primefac(A+1))
d = divisors(sorted(factors1+factors2))
for i in d:
if i*i > N: continue
j = N//i
x,y = (j+i)//2, (j-i)//2
a,b = (x-A)//2, (y+1)//2
if A//10 <= a < A and 0 <= b < A:
n = a*A+b
assert(n == b*b-a*a) # Check our logic
yield n

def divisors(factorlist):
"""Generate all divisors of number from list of prime factors"""
factors = multiset(factorlist)
nfactors = len(factors)
a = [0]*nfactors; b = [1]*nfactors
yield 1
while True:
i = 0
while i < nfactors:
if a[i] < factors[i][1]: break
a[i] = 0; b[i] = 1; i += 1
if i == nfactors: break
a[i] += 1; b[i] *= factors[i][0]
for j in range(0,i): b[j] = b[i]
yield b[0]

def multiset(s):
"""Create a multiset from a (sorted) list of items"""
m = []; n = s[0]; count = 1
for i in range(1,len(s)):
if s[i] != n:
m.append((n,count))
n = s[i]
count = 1
else:
count += 1
m.append((n,count))
return m

for n in range(2,1000,2):
for m in excellent(n//2): print m
```

The counts for numbers up to 10100:

```2: count = 1 factors = [3, 3, 11]
4: count = 1 factors = [3, 3, 11, 101]
6: count = 8 factors = [3, 3, 3, 7, 11, 13, 37]
8: count = 3 factors = [3, 3, 11, 73, 101, 137]
10: count = 3 factors = [3, 3, 11, 41, 271, 9091]
12: count = 13 factors = [3, 3, 3, 7, 11, 13, 37, 101, 9901]
14: count = 2 factors = [3, 3, 11, 239, 4649, 909091]
16: count = 3 factors = [3, 3, 11, 17, 73, 101, 137, 5882353]
18: count = 28 factors = [3, 3, 3, 3, 7, 11, 13, 19, 37, 52579, 333667]
20: count = 15 factors = [3, 3, 11, 41, 101, 271, 3541, 9091, 27961]
22: count = 9 factors = [3, 3, 11, 11, 23, 4093, 8779, 21649L, 513239L]
24: count = 51 factors = [3, 3, 3, 7, 11, 13, 37, 73, 101, 137, 9901, 99990001]
26: count = 5 factors = [3, 3, 11, 53, 79, 859, 265371653, 1058313049]
28: count = 17 factors = [3, 3, 11, 29, 101, 239, 281, 4649L, 909091L, 121499449]
30: count = 435 factors = [3, 3, 3, 7, 11, 13, 31, 37, 41, 211, 241, 271, 2161, 9091, 2906161]
32: count = 157 factors = [3, 3, 11, 17, 73, 101, 137, 353, 449, 641, 1409, 69857, 5882353]
34: count = 4 factors = [3, 3, 11, 103, 4013L, 2071723L, 5363222357L, 21993833369L]
36: count = 66 factors = [3, 3, 3, 3, 7, 11, 13, 19, 37, 101, 9901L, 52579L, 333667L, 999999000001L]
38: count = 2 factors = [3, 3, 11, 909090909090909091L, 1111111111111111111L]
40: count = 103 factors = [3, 3, 11, 41, 73, 101, 137, 271, 3541L, 9091L, 27961L, 1676321L, 5964848081L]
42: count = 999 factors = [3, 3, 3, 7, 7, 11, 13, 37, 43, 127, 239, 1933L, 2689L, 4649L, 459691L, 909091L, 10838689L]
44: count = 89 factors = [3, 3, 11, 11, 23, 89, 101, 4093L, 8779L, 21649L, 513239L, 1052788969L, 1056689261L]
46: count = 2 factors = [3, 3, 11, 47, 139, 2531L, 549797184491917L, 11111111111111111111111L]
48: count = 188 factors = [3, 3, 3, 7, 11, 13, 17, 37, 73, 101, 137, 9901L, 5882353L, 99990001L, 9999999900000001L]
50: count = 45 factors = [3, 3, 11, 41, 251, 271, 5051L, 9091L, 21401L, 25601L, 182521213001L, 78875943472201L]
52: count = 11 factors = [3, 3, 11, 53, 79, 101, 521, 859, 265371653L, 1058313049L, 1900381976777332243781L]
54: count = 150 factors = [3, 3, 3, 3, 3, 7, 11, 13, 19, 37, 757, 52579L, 333667L, 70541929L, 14175966169L, 440334654777631L]
56: count = 99 factors = [3, 3, 11, 29, 73, 101, 137, 239, 281, 4649L, 7841L, 909091L, 121499449L, 127522001020150503761L]
58: count = 2 factors = [3, 3, 11, 59, 3191L, 16763L, 43037L, 62003L, 77843839397L, 154083204930662557781201849L]
60: count = 35929 factors = [3, 3, 3, 7, 11, 13, 31, 37, 41, 61, 101, 211, 241, 271, 2161L, 3541L, 9091L, 9901L, 27961L, 2906161L, 4188901L, 39526741L]
62: count = 2 factors = [3, 3, 11, 2791L, 6943319L, 57336415063790604359L, 909090909090909090909090909091L]
64: count = 1162 factors = [3, 3, 11, 17, 73, 101, 137, 353, 449, 641, 1409L, 19841L, 69857L, 976193L, 5882353L, 6187457L, 834427406578561L]
66: count = 478 factors = [3, 3, 3, 7, 11, 11, 13, 23, 37, 67, 4093L, 8779L, 21649L, 513239L, 599144041L, 183411838171L, 1344628210313298373L]
68: count = 28 factors = [3, 3, 11, 101, 103, 4013L, 2071723L, 28559389L, 1491383821L, 5363222357L, 21993833369L, 2324557465671829L]
70: count = 146 factors = [3, 3, 11, 41, 71, 239, 271, 4649L, 9091L, 123551L, 909091L, 4147571L, 102598800232111471L, 265212793249617641L]
72: count = 3627 factors = [3, 3, 3, 3, 7, 11, 13, 19, 37, 73, 101, 137, 3169L, 9901L, 52579L, 98641L, 333667L, 99990001L, 999999000001L, 3199044596370769L]
74: count = 4 factors = [3, 3, 11, 7253L, 2028119L, 247629013L, 422650073734453L, 296557347313446299L, 2212394296770203368013L]
76: count = 5 factors = [3, 3, 11, 101, 722817036322379041L, 909090909090909091L, 1111111111111111111L, 1369778187490592461L]
78: count = 700 factors = [3, 3, 3, 7, 11, 13, 13, 37, 53, 79, 157, 859, 6397L, 216451L, 265371653L, 1058313049L, 388847808493L, 900900900900990990990991L]
80: count = 605 factors = [3, 3, 11, 17, 41, 73, 101, 137, 271, 3541L, 9091L, 27961L, 1676321L, 5070721L, 5882353L, 5964848081L, 19721061166646717498359681L]
82: count = 2 factors = [3, 3, 11, 83, 1231L, 538987L, 2670502781396266997L, 3404193829806058997303L, 201763709900322803748657942361L]
84: count = 59490 factors = [3, 3, 3, 7, 7, 11, 13, 29, 37, 43, 101, 127, 239, 281, 1933L, 2689L, 4649L, 9901L, 226549L, 459691L, 909091L, 10838689L, 121499449L, 4458192223320340849L]
86: count = 9 factors = [3, 3, 11, 173, 1527791L, 57009401L, 2182600451L, 1963506722254397L, 2140992015395526641L, 7306116556571817748755241L]
88: count = 105 factors = [3, 3, 11, 11, 23, 73, 89, 101, 137, 617, 4093L, 8779L, 21649L, 513239L, 1052788969L, 1056689261L, 16205834846012967584927082656402106953L]
90: count = 50344 factors = [3, 3, 3, 3, 7, 11, 13, 19, 31, 37, 41, 211, 241, 271, 2161L, 9091L, 29611L, 52579L, 238681L, 333667L, 2906161L, 3762091L, 8985695684401L, 4185502830133110721L]
92: count = 26 factors = [3, 3, 11, 47, 101, 139, 1289L, 2531L, 18371524594609L, 549797184491917L, 11111111111111111111111L, 4181003300071669867932658901L]
94: count = 2 factors = [3, 3, 11, 6299L, 35121409L, 4855067598095567L, 297262705009139006771611927L, 316362908763458525001406154038726382279L]
96: count = 80002 factors = [3, 3, 3, 7, 11, 13, 17, 37, 73, 97, 101, 137, 353, 449, 641, 1409L, 9901L, 69857L, 206209L, 5882353L, 99990001L, 66554101249L, 75118313082913L, 9999999900000001L]
98: count = 10 factors = [3, 3, 11, 197, 239, 4649L, 909091L, 505885997L, 1976730144598190963568023014679333L, 5076141624365532994918781726395939035533L]
100: count = 3573 factors = [3, 3, 11, 41, 101, 251, 271, 3541L, 5051L, 9091L, 21401L, 25601L, 27961L, 60101L, 7019801L, 182521213001L, 14103673319201L, 78875943472201L, 1680588011350901L]
```

Time to generate that lot, about 3m30s on my Core I3 laptop, about 30m on my Raspberry Pi 2.

Finally, if we want really big numbers, then we don’t need a full factorization – http://stdkmd.com/nrr/repunit/ has enough factors of 102016-1, for example, to find, among others:

```467203616037752709753640875404905278610286278867781588976105221346970432
395736683257666616868811083043941463314513845761368180251295614288295272
712974920189045619317506423133721608367014923830041699210760183164585217
599174938750569917513095900320978144876083087591215818424979192187341459
756509047543186958692244904217361382312220589759551138455399864423950556
877618463844146927376784311673205223822619959320039184981861810037019868
259785305305318574127400789513920685551635257636719954377249042716215901
383844447790548649266546486400561808622649593166435681150190744136685886
335659851446510906275097594875425811830427470257238967118169107518206073
615596540306297513679739458887733205329861379807932402155440131595447258
905459620119681006553819769296088325096562295835757909167184772591564873
889571115660015460170065915133627063917081464432904541564462471704065596
995864597351005042459541065531684772723537478273137238536882143270837967
355784692820692700257668090096174851711901872379544776234666991080481050
827938907695794044434449897483410036041789283630321915114061710879582491
425436499562245749681317500307257068229179611956588865266983050266693593
782449462829670744802988126608915433835744545694648340583599198978087691
600147477867595689158751559170609174089828925445708502246431435332615649
355141085750085715940292846105899585176839916452603275591677348739914677
778216128911941208439459006047576543241292648992222090990416741035195043
278539082418243317317374583211686841192215307443355453487377377290350196
371354628663013301973808912217716856093563005467214390853254407353722627
416963492751125708925240306094459245276161787679224919637918913006752210
513662791886758732646236407566089976349268846643228836678505302621204788
560791609138040737880910308235667105956827669559476643173829425259196119
155907391268256981917731226756506952400793923896521065760681749285568778
344719401424538913519492510286853888028737195080140956569785690813785590
037948199410250551049984142378021668001361835139075663440830359075663125
```

Which should be enough excellence for anybody. If not, https://www.dropbox.com/s/9xdnxd0ifla0zhf/excellent100.txt.gz has a list of all numbers up to 100 digits.