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A197928 Number of pairs of integers i,j with 1<=i<=n, 1<=j<=i, such that i^2-j^2 = i*j (mod n). 1
1, 1, 1, 3, 3, 1, 1, 3, 6, 3, 11, 3, 1, 1, 3, 10, 1, 6, 19, 11, 1, 11, 1, 3, 15, 1, 6, 3, 29, 3, 31, 10, 11, 1, 3, 21, 1, 19, 1, 11, 41, 1, 1, 43, 24, 1, 1, 10, 28, 15, 1, 3, 1, 6, 53, 3, 19, 29, 59, 11, 61, 31, 6, 36, 3, 11, 1, 3, 1, 3, 71, 21, 1, 1, 15, 75, 11, 1, 79, 42, 45, 41, 1, 3, 3, 1, 29, 43, 89, 24, 1, 3, 31, 1, 93, 10, 1, 28, 96, 55, 101, 1, 1, 3, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

It appears that, except for the first term, a(n)=n if and only if n is a prime congruent to 1 or 4 (mod 5).

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..1000

MAPLE

a:= n-> add(add(`if`(irem((i-j)*(i+j)-i*j, n)=0, 1, 0), j=1..i), i=1..n):

seq(a(n), n=1..100);  # Alois P. Heinz, Oct 19 2011

CROSSREFS

Cf. A180783.

Sequence in context: A266539 A090569 A160324 * A109439 A247646 A133333

Adjacent sequences:  A197925 A197926 A197927 * A197929 A197930 A197931

KEYWORD

nonn

AUTHOR

John W. Layman, Oct 19 2011

STATUS

approved

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Last modified July 8 08:08 EDT 2020. Contains 335520 sequences. (Running on oeis4.)