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A231791
Integers k such that A231589(k) = floor(k*(k-1)/4) - k.
1
8, 25, 77, 125, 133, 209, 301, 325, 425, 469, 473, 725, 737, 817, 925, 1025, 1141, 1273, 1325, 1525, 1625, 1793, 1825, 2125, 2225, 2425, 2525, 2725, 2825, 2881, 3097, 3425, 3625, 3725, 3925, 4325, 4525, 4625, 4825, 4925, 5125, 5525, 5725, 5825, 6025, 6425
OFFSET
1,1
COMMENTS
It appears that this sequence is the union of 3 sets.
First term is 8, and is the only even known value.
Then we get terms that are equal to 25 * b with b a squarefree product of primes congruent to 1 modulo 4 (A002144), that is, terms of A231754.
And we get the following terms 77, 133, 209, 301, 469, 473, 737, 817, 1141, 1273, 1793, 2881, 3097, 7009, 10921. These numbers are the products of 2 distinct primes from this list: 7, 11, 19, 43, 67, 163 (a subsequence of A003173).
LINKS
S. A. Shirali, A family portrait of primes-a case study in discrimination, Math. Mag. Vol. 70, No. 4 (Oct., 1997), pp. 263-272.
PROG
(PARI) isok(n) = A231589(n) == n*(n-1)/4 - n;
CROSSREFS
Cf. A231589.
Sequence in context: A287120 A127813 A295911 * A035073 A041120 A042637
KEYWORD
nonn
AUTHOR
Michel Marcus, Nov 13 2013
STATUS
approved