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A069741 Let M_n be the n X n matrix M_(i,j)=1/(2^i+2^j), then a(n) is the numerator of det(M_n). 1
1, 1, 1, 49, 2401, 113060689, 260871824431729, 9708455965188246321478801, 361304320362377236050632364626862769, 3511057522394397982450601057907077808699210592028881 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

a(n) seems always to be a square and 7 seems to follow a rule in a(n) factorization. Maximal k such that 7^k divides a(n) are 0, 0, 0, 2, 4, 6, 10, 14, 18, 24, 30, 36, 44, 52, 60, 70, 80, 90, 102, 114, 126, 142, 158, 174, 192... Hence if b(n)=maximum exponent of 7 in factorization of a(n), b(3n+1)=A049450(n); b(3n+2)=A049450(n)+2*n; b(3n+3)=A049450(n)+4n

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..25

PROG

(PARI) for(n=1, 70, print1(numerator(matdet(matrix(n, n, i, j, 1/(2^i+2^j)))), ", "))

CROSSREFS

Cf. A069743.

Sequence in context: A218751 A120999 A087752 * A203384 A099367 A307811

Adjacent sequences:  A069738 A069739 A069740 * A069742 A069743 A069744

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Apr 21 2002

STATUS

approved

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Last modified September 26 10:50 EDT 2022. Contains 356997 sequences. (Running on oeis4.)