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A333976 Number of pairs of divisors of n, (d1,d2), such that d1 <= d2 and gcd(d1,d2) > 1. 1
0, 1, 1, 3, 1, 5, 1, 6, 3, 5, 1, 13, 1, 5, 5, 10, 1, 13, 1, 13, 5, 5, 1, 25, 3, 5, 6, 13, 1, 22, 1, 15, 5, 5, 5, 32, 1, 5, 5, 25, 1, 22, 1, 13, 13, 5, 1, 41, 3, 13, 5, 13, 1, 25, 5, 25, 5, 5, 1, 55, 1, 5, 13, 21, 5, 22, 1, 13, 5, 22, 1, 60, 1, 5, 13, 13, 5, 22, 1, 41, 10 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

Table of n, a(n) for n=1..81.

FORMULA

a(n) = Sum_{d1|n, d2|n, d1<=d2} (1-[gcd(d1,d2) = 1]), where [ ] is the Iverson bracket.

EXAMPLE

a(7) = 1; (7,7)

a(8) = 6; (2,2), (2,4), (2,8), (4,4), (4,8), (8,8)

a(9) = 3; (3,3), (3,9), (9,9)

a(10) = 5; (2,2), (2,10), (5,5), (5,10), (10,10)

MATHEMATICA

Table[Sum[Sum[(1 - KroneckerDelta[GCD[i, k], 1]) (1 - Ceiling[n/k] + Floor[n/k]) (1 - Ceiling[n/i] + Floor[n/i]), {i, k}], {k, n}], {n, 100}]

CROSSREFS

Cf. A275387 (same with d1<d2), A337298.

Sequence in context: A324896 A029652 A238952 * A129510 A225656 A087913

Adjacent sequences:  A333973 A333974 A333975 * A333977 A333978 A333979

KEYWORD

nonn

AUTHOR

Wesley Ivan Hurt, Jan 01 2021

STATUS

approved

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Last modified January 26 21:14 EST 2022. Contains 350600 sequences. (Running on oeis4.)