Oasis, 4 bytes

###Code:

nj+U

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###Explanation:

Extended version:

nj+00
 0 = a(0)
 0 = a(1)
a(n) =
n # Push n
 j # Get the largest divisor under n
 + # Add to a(n - 1)

Oasis, 4 bytes

Code:

nj+U

Try it online!

Explanation:

Extended version:

nj+00
 0 = a(0)
 0 = a(1)
a(n) =
n # Push n
 j # Get the largest divisor under n
 + # Add to a(n - 1)

Oasis, 4 bytes

Code:

nj+U

Try it online!

Oasis, 4 bytes

###Code:

nj+U

Try it online!

###Explanation:

Extended version:

nj+00
 0 = a(0)
 0 = a(1)
a(n) =
n # Push n
 j # Get the largest divisor under n
 + # Add to a(n - 1)