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A333626
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Number of ways to write n as the sum of two positive integers that have the same number of divisors.
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3
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0, 1, 0, 1, 1, 1, 1, 2, 1, 2, 0, 2, 2, 3, 1, 4, 0, 4, 1, 4, 2, 4, 1, 5, 2, 3, 1, 4, 3, 6, 2, 6, 2, 6, 2, 8, 2, 3, 2, 7, 4, 7, 4, 7, 3, 6, 3, 12, 5, 7, 0, 7, 4, 10, 3, 8, 3, 5, 3, 13, 7, 6, 4, 11, 6, 11, 3, 8, 3, 11, 3, 17, 8, 7, 4, 9, 6, 12, 5, 12, 4, 9, 5
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OFFSET
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1,8
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LINKS
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FORMULA
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a(n) = Sum_{i=1..floor(n/2)} [d(i) = d(n-i)], where [] is the Iverson bracket and d is the number of divisors of n (A000005).
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EXAMPLE
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a(14) = 3; There are 3 ways to write 14 as the sum of two numbers with the same number of divisors: 14 = 11+3 (11 and 3 both have 2 divisors), 14 = 8+6 (8 and 6 both have 4 divisors), 14 = 7+7 (7 has 2 divisors).
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MATHEMATICA
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Table[Sum[KroneckerDelta[DivisorSigma[0, i], DivisorSigma[0, n - i]], {i, Floor[n/2]}], {n, 100}]
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PROG
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(PARI) a(n) = sum(i=1, n\2, numdiv(i) == numdiv(n-i)); \\ Michel Marcus, Mar 30 2020
(PARI) first(n) = {my(res, v, c); a2182inv = 128; res = vector(n); res[2] = 1; my(v = List(vector(a2182inv, i, List()))); for(i = 2, n, c = numdiv(i); for(i = #v + 1, c, listput(v, List()); ); listput(v[c], i) ); for(i = 1, a2182inv, for(j = 1, #v[i], for(k = j, #v[i], c = v[i][j] + v[i][k]; if(c <= n, res[c]++ ) ) ) ); res } \\ David A. Corneth, Apr 20 2020
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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