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A005245
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Complexity of n: number of 1's required to build n using + and *.
(Formerly M0457)
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25
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1, 2, 3, 4, 5, 5, 6, 6, 6, 7, 8, 7, 8, 8, 8, 8, 9, 8, 9, 9, 9, 10, 11, 9, 10, 10, 9, 10, 11, 10, 11, 10, 11, 11, 11, 10, 11, 11, 11, 11, 12, 11, 12, 12, 11, 12, 13, 11, 12, 12, 12, 12, 13, 11, 12, 12, 12, 13, 14, 12, 13, 13, 12, 12, 13, 13, 14, 13, 14, 13, 14, 12, 13, 13, 13, 13, 14, 13, 14
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OFFSET
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1,2
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COMMENTS
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The complexity of an integer n is the least number of 1's needed to represent it using only additions, multiplications and parentheses. This does not allow juxtaposition of 1's to form larger integers, so for example, 2 = 1+1 has complexity 2, but 11 does not ("pasting together" two 1's is not an allowed operation).
The complexity of a number has been defined in several different ways by different authors. See the Index to the OEIS for other definitions.
Guy asks if a(p) = a(p-1) + 1 for prime p. Martin Fuller found the least counterexample p = 353942783 in 2008, see Fuller link. - Charles R Greathouse IV, Oct 04 2012
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REFERENCES
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M. Criton, "Les uns de Germain", Problem No.4 pp 13 ; 68 in '7 x 7 Enigmes Et Defis Mathematiques pour tous', vol.25 Editions POLE Paris 2001.
J. Arias de Reyna, Complejidad de los numeros naturales, Gaceta de la Real Sociedad Matematica Espanola, 3, (2000), 230-250.
R. K. Guy, Some suspiciously simple sequences, Amer. Math. Monthly 93 (1986), 186-190; 94 (1987), 965; 96 (1989), 905.
R. K. Guy, Unsolved Problems Number Theory, Sect. F26.
K. Mahler and J. Popken, Over een Maximumprobleem uit de Rekenkunde (Dutch: On a maximum problem in arithmetic), Nieuw Arch. Wiskunde, (3) 1 (1953), pp. 1-15.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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M. F. Hasler, Table of n, a(n) for n = 1..10000
Harry Altman, Highest few sums and products of ones
Harry Altman and Joshua Zelinsky, Numbers with integer complexity close to the lower bound, arXiv:1207.4841
Martin N. Fuller, C program
Janis Iraids, Online calculator of a(n) for n < 10^12
Jānis Iraids, Kaspars Balodis, Juris Čerņenoks, Mārtiņš Opmanis, Rihards Opmanis, and Kārlis Podnieks, Integer complexity: experimental and analytical results (2012)
Daniel A. Rawsthorne, How many 1's are needed?, Fibonacci Quarterly 27 (1989), pp. 14-17.
Srinivas Vivek V. and Shankar B. R., Integer Complexity: Breaking the Theta(n^2) barrier, World Academy of Science 41 (2008), pp. 690-691.
Venecia Wang, A counterexample to the prime conjecture of expressing numbers using just ones, Journal of Number Theory, submitted. video abstract
Eric Weisstein's World of Mathematics, Integer Complexity
Index to sequences related to the complexity of n
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FORMULA
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It's known that a(n)<= A061373(n) but I think 0 <= A061373(n)-a(n) <= 1 also holds. - Benoit Cloitre, Nov 23 2003. That's false: the numbers {46, 235, 649, 1081, 7849, 31669, 61993} require {1, 2, 3, 4, 5, 6, 7} fewer 1's in A005245 than in A061373. - Ed Pegg Jr, Apr 13 2004.
Coppersmith proved that 3 log_3 n <= a(n) <= 3 log_2 n for all n > 1. - Charles R Greathouse IV, Apr 19 2012
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EXAMPLE
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Contribution from Lekraj Beedassy, Jul 04 2009: (Start)
n ........... minimal expression........... a(n)= number of 1's
1.....................1.......................1
2....................1+1......................2
3...................1+1+1.....................3
4................(1+1)*(1+1)..................4
5...............(1+1)*(1+1)+1.................5
6...............(1+1)*(1+1+1).................5
7..............(1+1)*(1+1+1)+1................6
8.............(1+1)*(1+1)*(1+1)...............6
9..............(1+1+1)*(1+1+1)................6
10............(1+1+1)*(1+1+1)+1...............7
11...........(1+1+1)*(1+1+1)+1+1..............8
12...........(1+1)*(1+1)*(1+1+1)..............7
13..........(1+1)*(1+1)*(1+1+1)+1.............8
14.........{(1+1)*(1+1+1)+1}*(1+1)............8
15.........{(1+1)*(1+1)+1}*(1+1+1)............8
16.........(1+1)*(1+1)*(1+1)*(1+1)............8
17........(1+1)*(1+1)*(1+1)*(1+1)+1...........9
18..........(1+1)*(1+1+1)*(1+1+1).............8
19.........(1+1)*(1+1+1)*(1+1+1)+1............9
20........{(1+1+1)*(1+1+1)+1}*(1+1)...........9
21........{(1+1)*(1+1+1)+1}*(1+1+1)...........9
22.......{(1+1)*(1+1+1)+1}*(1+1+1)+1..........10
23......{(1+1)*(1+1+1)+1}*(1+1+1)+1+1.........11
24........(1+1)*(1+1)*(1+1)*(1+1+1)...........9
25.......(1+1)*(1+1)*(1+1)*(1+1+1)+1..........10
26......{(1+1)*(1+1)*(1+1+1)+1}*(1+1).........10
27.........(1+1+1)*(1+1+1)*(1+1+1)............9
28........(1+1+1)*(1+1+1)*(1+1+1)+1...........10
29.......(1+1+1)*(1+1+1)*(1+1+1)+1+1..........11
30.......{(1+1+1)*(1+1+1)+1}*(1+1+1)..........10
31......{(1+1+1)*(1+1+1)+1}*(1+1+1)+1.........11
32......(1+1)*(1+1)*(1+1)*(1+1)*(1+1).........10
33.....(1+1)*(1+1)*(1+1)*(1+1)*(1+1)..........11
34...{(1+1)*(1+1)*(1+1)*(1+1)+1}*(1+1)........11
................................................
(End)
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MAPLE
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with (numtheory):
a:= proc(n) option remember;
`if`(n=1, 1, min (seq (a(i)+a(n-i), i=1..n/2),
seq (a(d)+a(n/d), d=divisors(n) minus {1, n})))
end:
seq (a(n), n=1..100); # Alois P. Heinz, Apr 18 2012
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PROG
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(PARI, from M. F. Hasler, Jan 30 2008) A005245(n /* start by calling this with the largest needed n */, lim/* see below */) = { local(d); n<6&return(n);
if(n<=#A05245, A05245[n]&return(A05245[n]) /* return memoized result if available */,
A05245=vector(n) /*create vector if needed - should better re-use exiting data if available */);
for(i=1, n-1, A05245[i] || A05245[i]=A005245(i, lim)); /* compute all previous elements */
A05245[n]=min( vecmin(vector(min(n\2, if(lim>0, lim, n)), k, A05245[k]+A05245[n-k])) /* additive possibilities - if lim>0 is given, consider a(k)+a(n-k) only for k<=lim - we know it is save to use lim=1 up to n=2e7 */, if( isprime(n), n, vecmin(vector((-1+#d=divisors(n))\2, i, A05245[d[i+1]]+A05245[d[ #d-i]]))/* multiplicative possibilities */))}
See also the Python program by Tim Peters at A005421.
(Haskell)
import Data.List (genericIndex)
a005245 n = a005245_list `genericIndex` (n-1)
a005245_list = 1 : f 2 [1] where
f x ys = y : f (x + 1) (y : ys) where
y = minimum $
(zipWith (+) (take (x `div` 2) ys) (reverse ys)) ++
(zipWith (+) (map a005245 $ tail $ a161906_row x)
(map a005245 $ reverse $ init $ a161908_row x))
-- Reinhard Zumkeller, Mar 08 2013
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CROSSREFS
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Cf. A000792 (largest integer of given complexity), A003313, A076142, A076091, A061373, A005421, A064097, A005520, A025280, A003037, A161906, A161908.
Sequence in context: A007600 A195872 A091333 * A061373 A104135 A046108
Adjacent sequences: A005242 A005243 A005244 * A005246 A005247 A005248
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KEYWORD
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nonn,nice
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AUTHOR
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N. J. A. Sloane.
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EXTENSIONS
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More terms from David W. Wilson May 15 1997
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STATUS
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approved
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