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 A048050 Chowla's function: sum of divisors of n except 1 and n. 81
 0, 0, 0, 2, 0, 5, 0, 6, 3, 7, 0, 15, 0, 9, 8, 14, 0, 20, 0, 21, 10, 13, 0, 35, 5, 15, 12, 27, 0, 41, 0, 30, 14, 19, 12, 54, 0, 21, 16, 49, 0, 53, 0, 39, 32, 25, 0, 75, 7, 42, 20, 45, 0, 65, 16, 63, 22, 31, 0, 107, 0, 33, 40, 62, 18, 77, 0, 57, 26, 73, 0, 122, 0, 39, 48, 63, 18, 89 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS a(n) = 0 if and only if n is a noncomposite number (Cf. A008578). - Omar E. Pol, Jul 31 2012 If n is semiprime, a(n) = A008472(n). - Wesley Ivan Hurt, Aug 22 2013 If n = p*q where p and q are distinct primes then a(n) = p+q. If k,m > 1 are coprime, then a(k*m) = a(k)*a(m) + (m+1)*a(k) + (k+1)*a(m) + k + m. - Robert Israel, Apr 28 2015 a(n) is also the total number of parts in the partitions of n into equal parts that contain neither 1 nor n as a part (see example). More generally, a(n) is the total number of parts congruent to 0 mod k in the partitions of k*n into equal parts that contain neither k nor k*n as a part. - Omar E. Pol, Nov 24 2019 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 M. Lal and A. Forbes, A note on Chowla's function, Math. Comp., 25 (1971), 923-925. FORMULA a(n) = A000203(n) - A065475(n). a(n) = A001065(n)-1, n>1. For n>1: a(n) = sum(A027750(n,k): k=2..A000005(n)-1). - Reinhard Zumkeller, Feb 09 2013 a(n) = A000203(n)-n-1, n>1. - Wesley Ivan Hurt, Aug 22 2013 G.f.: Sum_{k>=2} k*x^(2*k)/(1 - x^k). - Ilya Gutkovskiy, Jan 22 2017 EXAMPLE For n = 20 the divisors of 20 are 1,2,4,5,10,20, so a(20) = 2+4+5+10 = 21. On the other hand, the partitions of 20 into equal parts that contain neither 1 nor 20 as a part are [10,10], [5,5,5,5], [4,4,4,4,4], [2,2,2,2,2,2,2,2,2,2]. There are 21 parts, so a(20) = 21. - Omar E. Pol, Nov 24 2019 MAPLE A048050 := proc(n) if n > 1 then numtheory[sigma](n)-1-n ; else 0; end if; end proc: MATHEMATICA f[n_]:=Plus@@Divisors[n]-n-1; Table[f[n], {n, 100}] (*Vladimir Joseph Stephan Orlovsky, Sep 13 2009*) Join[{0}, DivisorSigma[1, #]-#-1&/@Range[2, 80]] (* Harvey P. Dale, Feb 25 2015 *) PROG (MAGMA) A048050:=func< n | n eq 1 or IsPrime(n) select 0 else &+[ a: a in Divisors(n) | a ne 1 and a ne n ] >; [ A048050(n): n in [1..100] ]; // Klaus Brockhaus, Mar 04 2011 (PARI) a(n)=if(n>1, sigma(n)-n-1, 0) \\ Charles R Greathouse IV, Nov 20 2012 (Haskell) a048050 1 = 0 a048050 n = (subtract 1) \$ sum \$ a027751_row n -- Reinhard Zumkeller, Feb 09 2013 (Python) from sympy import divisors def a(n): return sum(divisors(n)[1:-1]) # Indranil Ghosh, Apr 26 2017 CROSSREFS Cf. A000203, A001065, A000593, A002954, A048995, A007956, A048671, A182936. Cf. A057533, A005276, A027751. Sequence in context: A104755 A242690 A054013 * A329375 A078153 A104035 Adjacent sequences:  A048047 A048048 A048049 * A048051 A048052 A048053 KEYWORD nonn,nice,easy AUTHOR STATUS approved

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Last modified July 3 10:20 EDT 2020. Contains 335417 sequences. (Running on oeis4.)