Need to devise a number crunching algorithm

Question

I stumbled upon this question:

7 power 7 is 823543. Which higher power of 7 ends with 823543 ?

How should I go about it ? The one I came up with is very slow, it keeps on multiplying by 7 and checks last 6 digits of the result for a match.

I tried with Lou's code:

int x=1;
    for (int i=3;i<=100000000;i=i+4){
            x=(x*7)%1000000;
            System.out.println("i="+ i+" x= "+x);
            if (x==823543){
                System.out.println("Ans "+i);}
            }

And CPU sounds like a pressure cooker but couldn't get the answer :(

Answer 1

Multiply modulo 10^6. See this Lua code.

Answer 2

Not so much an answer, more a hint:

Observe that the pattern of rightmost digits of powers of 7 goes 1,7,9,3,1,7,9,3,1,7,... so you only need to generate every 4th power of 7 from the 3rd. Further study might show a pattern for the two (three, four, ...) rightmost digits, but I haven't done studied them for you.

Be prepared for some very large numbers, Mathematica reports that the next power of 7 with the sought-for rightmost digits is the 5007th.

Which I guess answers your question -- a faster approach is to post on SO and wait for someone to tell you the answer ! You could even try Wolfram Alpha if you don't like the SO algorithm.

Answer 3

Another hint: You are only interested in the last N digits: you can perform calculations modulo 10^N and keep the result fit nicely into an integer

Answer 4

You could use Euler's generalization of Fermat's little theorem which applied to your case says that for any number a that is not divisible by two or five, a to the power 400000 is equal to 1 modulo 10^6. Which means that 7^400000 is equal to one and 7^400007 is equal to 823543 modulo 10^6

There may be smaller powers of 7 that are also equal to one modulo 10^6. Any such power should be a divisor of 400000. So if you search all divisors of 400000 you should find your answer.

Answer 5

Brute-force solution in Python:

def check():
    i = 8
    while True:
        if str(7**i)[-6:] == "823543":
            print i, 7**i
            break
        i += 1

if __name__ == "__main__":
  check()

Runs in a tad more then 10 seconds on my machine:

$ time python 7\*\*7.py 
5007 25461638709540284156782446957365168367138070393489656084508152816071765490828583739345420574947301301356529652113030016806506783009529977928336772622054260724106711204039012806363481521302203821096274017061906820115931889920385802499836705571461280700786627503189500663279772123190279763997339608040949194040289041117811256914511855302927500076094761237077649092849658261309060277197389760351907599243227298336309204635761799394324969277220810221310805265921431367291459357151617279190810954501590069774137519833706444943573459910208627100504003480684029216932299650285683013274883359754231186580602570771682084721896446416234857382909168309309630688331305154545352580787700878011742720440707156231891841057992434850068501355342227582074144717324718396296563918284728120322255330707786227631084119636101174217518654320128390401231343058708073280898554293777842571799775647325449392944570725467462072394864457569308219294304248413378339223195121800534783732295135735588409249690562213409520783181313960347723827308102920022860541043691808218543350580271593107019737918976365348051012746817678592727727988993175444584453532474156202438866838819565827414970029052602274354173178190323239427022953097424087683011937130778414189673555875258508014323428137406618951161046883845551087123412471364400737145434714864392224194773030522352601143771552489895838728148761974811275894561985163094852437703080985644653666048979901975905667811053289029958524703063742007291722490298429637413913574845245364780928447142275001431370017543206188428912106120676556219532197108435997375879569102044435752697298456147512203108094030745606163915437604076966518127099543894645297945324345093247636119593298654296614887389164509070158924404441687810434488061150620012547321097786493748417764592151734279632949607485719050349385098350202294648324398902047614892248381794929374952059877187100434970751833289677556040879755065563758085919673107576808662549999202791489324437408075089456174056904323973798979207791446889016369166632636035638123394649891606479407561222474471530411700646266636732205895085248823824764170316644547100628119484733814900100986786082211477261114056206393554335903410036064553032366200714266053598548735147707681592574886559888869327771461046450774938490837810526377213647071217152427693219479552580138352651791476758476864761332281826701978038126122728967682552206820425685782165630494478519812498630475776384700259524274670258777572341538755828794632819515842335609785884327007667337426644594091547392441314523035569100326662245947022517857248412004291423280879791576077952474202068318934524092750814844945529148131063116233331840380254781283689084385600858175504170157015630699919186013526052643206240745256569669847298952477441594748635701081031979500954081732722211598460098426985932512920424237248250698541558227081975966598720056015879151923686438360541128221854058867910136449528237543680180470919685862102358708465872395643586424250239281775923511452769821487580471289910257908740451431952197725174728917413539539795856895884961513784804247268727165303942024508367184898248006123651950710237279288601317817391869969699767431782664773248447758526620050588927086506013616563459173620496200348863132442180734592661348887012997849309740799709045762939781801481205704629203758859772476278892928066844445088880207986848424855774325574728566649552154520262460969975214802828263093097997124519153537792591659204109087699977445745067857471581656151077039286563447099850537157044829081400190710358959493358343935904669416958301921942118288210835104022359479660409954097409669785908666166908117346073702337825511531650740900904200220658196171839969860945908503151878488455004283026700303698398069644419655035582904253655945381261383285097911378914794161551292914993411444083214513058414480129560671193659591364146612550890288116403596333209446976453193340267725222134755872075133141618388704912211996423838163706006930973361661094103734887312836613195349528793780496172839376426055357343094188450140671138356505144988151110902047791487250988374130384058324229250761311655685931891857894126054047458969174494155762486464149775147410127618088224310828566286409277000561087588768230619606746804073498788244935099280434916850444895829823543

real    0m10.779s
user    0m10.709s
sys 0m0.024s

Answer 6

The Fermat's little theorem approach is a mathematically sensible one, and just mulitplying over and over by 7 mod 10^6 is the simplest code, but there's another approach you could take that is computationally efficient (but requires more complex code). First, note that when multiplying by 7 the last digit depends only on the last digit before (i.e. we're doing everything mod 10). We multiply repeatedly by 7 to get

7  (4)9  (6)3  (2)1 (0)7 ...

Okay, great, so if we want a 3, we start at 7^3 and go up every 7^4 from there. Now, we note that when multiplying by 7^4, the last two digits depend only on the last two digits of 7^4 and the last two digits of the previous answer. 7^4 is 2401. So in fact the last two digits will always be the same when going up by 7^4.

What about the last three? Well, 7^3 = 343 and 7^4 ends with 401, so mod 1000 we get

343 543 743 943 143 343

We've got our first three digits in column #2 (543), and we see that the the sequence repeats ever 5, so we should go up from there by 7^20.

We can play this trick over and over again: find how often the next block of digits repeats, find the right subsequence within that block, and then multiply up not by 7 but by 7^n.

What we're really doing is finding a (multiplicative) ring over the m'th digit, and then multiplying the sizes of all the rings together to get the span between successive powers that have the same N digits if we follow this method. Here's some Scala code (2.8.0 Beta1) that does just this:

def powRing(bigmod: BigInt, checkmod: BigInt, mul: BigInt) = {
  val powers = Stream.iterate(1:BigInt)(i => (i*mul)%bigmod)
  powers.take( 2+powers.tail.indexWhere(_ % checkmod == 1) ).toList
}
def ringSeq(digits: Int, mod: BigInt, mul: BigInt): List[(BigInt,List[BigInt])] = {
  if (digits<=1) List( (10:BigInt , powRing(mod,10,mul)) )
  else {
    val prevSeq = ringSeq(digits-1, mod, mul)
    val prevRing = prevSeq.head
    val nextRing = powRing(mod,prevRing._1*10,prevRing._2.last)
    (prevRing._1*10 , nextRing) :: prevSeq
  }
}
def interval(digits: Int, mul: Int) = {
  val ring = ringSeq(digits, List.fill(digits)(10:BigInt).reduceLeft(_*_), mul)
  (1L /: ring)((p,r) => p * (r._2.length-1))
}

So, if we've found one case of the digits that we want, we can now find all of them by finding the size of the appropriate ring. In our case, with 6 digits (i.e. mod 10^6) and base 7, we find a repeat size of:

scala> interval(6,7)                                                           
res0: Long = 5000

So, we've got our answer! 7^7 is the first, 7^5007 is the second, 7^10007 is the third, etc..

Since this is generic, we can try other answers...11^11 = 285311670611 (an 8 digit number). Let's look at the interval:

scala> interval(12,11)            
res1: Long = 50000000000

So, this tells us that 11^50000000007 is the next number after 11^11 with the same initial set of 12 digits. Check by hand if you're curious!

Let's also check with 3^3--what's the next power of 3 whose decimal expansion ends with 27?

scala> interval(2,3)
res2: Long = 20

Should be 3^23. Checking:

scala> List.fill(23)(3L).reduceLeft((l,r) => {println(l*r) ; l*r})
9
27
81
243
729
2187
6561
19683
59049
177147
531441
1594323
4782969
14348907
43046721
129140163
387420489
1162261467
3486784401
10460353203
31381059609
94143178827

Yup!

---

(Switched code in edits to use BigInt so it could handle arbitrary numbers of digits. The code doesn't detect degenerate cases, though, so make sure you use a prime for the power....)