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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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS 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 Cf. A000188, A064276, A059306, A062801, A059976, A039623. Sequence in context: A310702 A310703 A298789 * A026372 A014690 A274383 Adjacent sequences:  A064365 A064366 A064367 * A064369 A064370 A064371 KEYWORD nonn AUTHOR Vladeta Jovovic, Sep 27 2001 STATUS approved

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Last modified August 8 19:29 EDT 2020. Contains 336298 sequences. (Running on oeis4.)