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A064368
Number of 2 X 2 symmetric singular matrices with entries from {0,...,n}.
1
1, 4, 7, 10, 15, 18, 21, 24, 29, 36, 39, 42, 47, 50, 53, 56, 65, 68, 75, 78, 83, 86, 89, 92, 97, 108, 111, 118, 123, 126, 129, 132, 141, 144, 147, 150, 163, 166, 169, 172, 177, 180, 183, 186, 191, 198, 201, 204, 213, 228, 239, 242, 247, 250, 257, 260, 265, 268
OFFSET
0,2
FORMULA
a(n) = n+1+2*Sum_{k=1..n} Sum_{d^2|k} phi(d), where phi = Euler totient function A000010.
PROG
(PARI) a(n) = n + 1 + 2*sum(k=1, n, sumdiv(k, d, issquare(d)*eulerphi(sqrtint(d)))) \\ Michel Marcus, Jun 17 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Sep 27 2001
STATUS
approved