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%I #35 Mar 23 2024 21:10:42
%S 1,2,2,4,2,4,2,6,4,4,2,7,2,4,4,9,2,7,2,7,4,4,2,10,4,4,6,7,2,8,2,11,4,
%T 4,4,12,2,4,4,10,2,8,2,7,7,4,2,14,4,7,4,7,2,10,4,10,4,4,2,13,2,4,7,15,
%U 4,8,2,7,4,8,2,16,2,4,7,7,4,8,2,14,9,4,2,13,4,4,4,10,2,13,4,7,4,4,4,17,2,7
%N Number of divisors of n, counting divisor multiplicity in n.
%C The multiplicity of a divisor d > 1 in n is defined as the largest power i for which d^i divides n; and for d = 1 it is defined as 1.
%C a(n) is also the sum of the multiplicities of the divisors of n.
%C In other words, a(n) = 1 + sum of the highest exponents e_i for which each number k_i in range 2 .. n divide n, as {k_i}^{e_i} | n. For nondivisors of n this exponent e_i is 0, for n itself it is 1. - _Antti Karttunen_, May 20 2017
%C From _Gus Wiseman_, Mar 25 2021: (Start)
%C Also the number of strict chains of divisors ending with n and having constant (equal) first quotients. The case starting with 1 is A089723. For example, the a(1) = 1 through a(12) = 7 chains are:
%C 1 2 3 4 5 6 7 8 9 10 11 12
%C 1|2 1|3 1|4 1|5 1|6 1|7 1|8 1|9 1|10 1|11 1|12
%C 2|4 2|6 2|8 3|9 2|10 2|12
%C 1|2|4 3|6 4|8 1|3|9 5|10 3|12
%C 2|4|8 4|12
%C 1|2|4|8 6|12
%C 3|6|12
%C (End)
%C a(n) depends only on the prime signature of n. - _David A. Corneth_, Mar 28 2021
%H Antti Karttunen, <a href="/A169594/b169594.txt">Table of n, a(n) for n = 1..10000</a>
%F From _Friedjof Tellkamp_, Feb 29 2024: (Start)
%F a(n) = A309891(n) + 1.
%F G.f.: x/(1-x) + Sum_{k>=2, j>=1} x^(k^j)/(1-x^(k^j)).
%F Dirichlet g.f.: zeta(s) * (1 + Sum_{k>=1} (zeta(k*s) - 1)).
%F Sum_{n>=1} a(n)/n^2 = (7/24) * Pi^2. (End)
%e The divisors of 8 are 1, 2, 4, 8 of multiplicity 1, 3, 1, 1, respectively. So a(8) = 1 + 3 + 1 + 1 = 6.
%t divmult[d_, n_] := Module[{output, i}, If[d == 1, output = 1, If[d == n, output = 1, i = 0; While[Mod[n, d^(i + 1)] == 0, i = i + 1]; output = i]]; output]; dmt0[n_] := Module[{divs, l}, divs = Divisors[n]; l = Length[divs]; Sum[divmult[divs[[i]], n], {i, 1, l}]]; Table[dmt0[i], {i, 1, 40}]
%t Table[1 + DivisorSum[n, IntegerExponent[n, #] &, # > 1 &], {n, 98}] (* _Michael De Vlieger_, May 20 2017 *)
%o (PARI)
%o A286561(n,k) = { my(i=1); if(1==k, 1, while(!(n%(k^i)), i = i+1); (i-1)); };
%o A169594(n) = sumdiv(n,d,A286561(n,d)); \\ _Antti Karttunen_, May 20 2017
%o (PARI) a(n) = { if(n == 1, return(1)); my(f = factor(n), u = vecmax(f[, 2]), cf = f, res = numdiv(f) - u + 1); for(i = 2, u, cf[, 2] = f[, 2]\i; res+=numdiv(factorback(cf)) ); res } \\ _David A. Corneth_, Mar 29 2021
%o (Scheme)
%o (define (A169594 n) (add (lambda (k) (A286561bi n k)) 1 n))
%o ;; Implements sum_{i=lowlim..uplim} intfun(i)
%o (define (add intfun lowlim uplim) (let sumloop ((i lowlim) (res 0)) (cond ((> i uplim) res) (else (sumloop (1+ i) (+ res (intfun i)))))))
%o ;; For A286561bi see A286561. - _Antti Karttunen_, May 20 2017
%o (Python)
%o def a286561(n, k):
%o i=1
%o if k==1: return 1
%o while n%(k**i)==0:
%o i+=1
%o return i-1
%o def a(n): return sum([a286561(n, d) for d in divisors(n)]) # _Indranil Ghosh_, May 20 2017
%Y Cf. A168512.
%Y Row sums of A286561, A286563 and A286564.
%Y A001055 counts factorizations (strict: A045778, ordered: A074206).
%Y A057567 counts chains of divisors with weakly increasing first quotients.
%Y A067824 counts strict chains of divisors ending with n.
%Y A253249 counts strict chains of divisors.
%Y A334997 counts chains of divisors of n by length.
%Y A342086 counts chains of divisors with strictly increasing first quotients.
%Y A342496 counts partitions with equal first quotients (strict: A342515, ranking: A342522, ordered: A342495).
%Y A342530 counts chains of divisors with distinct first quotients.
%Y Cf. A003238, A003242, A069916, A122651, A309891, A318991, A318992, A325545.
%K nonn,easy
%O 1,2
%A _Joseph L. Pe_, Dec 02 2009
%E Extended by _Ray Chandler_, Dec 08 2009
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