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A066907 Number of elements in GL(2,Z_n) x with x^2 == I mod n where I is the identity matrix. 5
1, 4, 14, 28, 32, 56, 58, 176, 110, 128, 134, 392, 184, 232, 448, 608, 308, 440, 382, 896, 812, 536, 554, 2464, 752, 736, 974, 1624, 872, 1792, 994, 2336, 1876, 1232, 1856, 3080, 1408, 1528, 2576, 5632, 1724, 3248, 1894, 3752, 3520, 2216, 2258, 8512, 2746 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) is multiplicative and for an odd prime power p^k : a(p^k) = 2 + p^(2k-1)(p+1). [corrected by Felix A. Pahl, Mar 08 2013]

Number of involutory matrices mod n. - Charles R Greathouse IV, May 29 2013

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A066947(n) + 1.

PROG

(PARI) a(n)=my(o=valuation(n, 2), f=factor(n>>o)); prod(i=1, #f[, 1], f[i, 1]^(2*f[i, 2])+f[i, 1]^(2*f[i, 2]-1)+2)*if(o, if(o>1, if(o>2, 9*4^(o-1)+32, 28), 4), 1) \\ Charles R Greathouse IV, May 29 2013

CROSSREFS

Cf. A066947, A060594.

Sequence in context: A094320 A200546 A304342 * A130439 A289179 A033690

Adjacent sequences:  A066904 A066905 A066906 * A066908 A066909 A066910

KEYWORD

nonn,mult,easy

AUTHOR

Sharon Sela (sharonsela(AT)hotmail.com), Jan 26 2002

EXTENSIONS

Added more terms (from A066947), Joerg Arndt, Mar 08 2013

STATUS

approved

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Last modified July 16 11:09 EDT 2020. Contains 335784 sequences. (Running on oeis4.)