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A025523
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a(n) = 1 + Sum_{ k < n and k | n} a(k).
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9
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1, 2, 3, 5, 6, 9, 10, 14, 16, 19, 20, 28, 29, 32, 35, 43, 44, 52, 53, 61, 64, 67, 68, 88, 90, 93, 97, 105, 106, 119, 120, 136, 139, 142, 145, 171, 172, 175, 178, 198, 199, 212, 213, 221, 229, 232, 233, 281, 283, 291, 294, 302, 303, 323, 326, 346, 349, 352, 353, 397, 398, 401
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OFFSET
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1,2
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COMMENTS
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Permanent of n X n (0,1) matrix defined by A(i,j)=1 iff j=1 or i divides j. - Vladeta Jovovic, Jul 05 2003
The subsequence of primes begins: 2, 3, 5, 19, 29, 43, 53, 61, 67, 97, 139, 199, 229, 233, 281, 283, 349, 353, 397, 401. - Jonathan Vos Post
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LINKS
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FORMULA
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a(n) = 1 + Sum_{k = 2 to n} a([n/k]). [Mitchell Lee (worthawholephan(AT)gmail.com), Jul 26 2010]
G.f. A(x) satisfies: A(x) = (1/(1 - x)) * (x + Sum_{k>=2} (1 - x^k) * A(x^k)). - Ilya Gutkovskiy, Aug 11 2021
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PROG
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(Python)
from functools import lru_cache
@lru_cache(maxsize=None)
if n == 0:
return 1
c, j = 2, 2
k1 = n//j
while k1 > 1:
j2 = n//k1 + 1
j, k1 = j2, n//j2
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CROSSREFS
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A173382 is an essentially identical sequence.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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