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A337557 Number of integers less than n with the same number of odd divisors as n. 1
0, 1, 0, 2, 1, 2, 3, 3, 0, 4, 5, 6, 7, 8, 0, 4, 9, 1, 10, 11, 1, 12, 13, 14, 2, 15, 2, 16, 17, 3, 18, 5, 4, 19, 5, 3, 20, 21, 6, 22, 23, 7, 24, 25, 0, 26, 27, 28, 4, 5, 8, 29, 30, 9, 10, 31, 11, 32, 33, 12, 34, 35, 1, 6, 13, 14, 36, 37, 15, 16, 38, 6, 39, 40, 2, 41, 17, 18, 42, 43 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,4
LINKS
FORMULA
a(n) = |{j < n : A001227(j) = A001227(n)}|.
EXAMPLE
a(10) = 4 because A001227(10) = 2 and also A001227(3) = A001227(5) = A001227(6) = A001227(7) = 2.
MATHEMATICA
Table[Length[Select[Range[n - 1], Sum[Mod[d, 2], {d, Divisors[#]}] == Sum[Mod[d, 2], {d, Divisors[n]}] &]], {n, 80}]
PROG
(PARI) a(n)={my(t=numdiv(n/2^valuation(n, 2))); sum(k=1, n-1, numdiv(k/2^valuation(k, 2))==t)} \\ Andrew Howroyd, Oct 31 2020
(PARI) first(n) = { my(m = Map(), res = vector(n)); for(i = 1, n, q = numdiv(i >> valuation(i, 2)); if(mapisdefined(m, q), res[i] = mapget(m, q); mapput(m, q, res[i]+1); , mapput(m, q, 1) ) ); res } \\ David A. Corneth, Oct 31 2020
CROSSREFS
Sequence in context: A054758 A077876 A095056 * A186333 A272917 A239627
KEYWORD
nonn,easy
AUTHOR
Ilya Gutkovskiy, Oct 31 2020
STATUS
approved

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Last modified March 28 14:38 EDT 2024. Contains 371254 sequences. (Running on oeis4.)