login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A259288 Odd numbers of the form (m*k)^2/(m^2-k^2) for distinct integers m and k. 1
147, 225, 405, 1323, 2025, 3645, 3675, 4225, 5625, 7203, 7623, 10125, 11025, 11907, 14415, 17457, 17787, 18225, 18513, 19845, 24375, 24843, 27225, 30625, 32805, 33075, 38025, 42483, 49005, 50625, 53067, 61347, 64827, 65025, 68445, 68607, 77763, 81225, 91125, 91875, 98397, 99225, 105625, 107163, 117045, 119025 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The first term ending in a 9 seems to be 1225449, and the first term ending in a 1 is 136161.

For 1 <= m <= 10^4 and 1 <= k <= m, there are 9217 numbers of the form (m*k)^2/(m^2-k^2). Of these numbers, only 679 are odd.

If a(n) is not a square, then m = 9*k or m = 7*k. If a(n) is a square, m does not appear to be a multiple of k.

Let a(n) be a square generated by m_1 and k_1. If a(n-1) is generated by m_2 and k_2, then k_1 = k_2 and m_1 < m_2.

The reciprocals of these numbers can be represented as the difference in the reciprocals of two squares (i.e., there exists two distinct integers m and k satisfying 1/a(n) = 1/m^2 - 1/k^2).

LINKS

Table of n, a(n) for n=1..46.

EXAMPLE

(84*12)^2/(84^2-12^2) = 84^2/48 = 147. So 147 is a member of this sequence. (Note that k=12 and m=84 and so m=7*k.)

PROG

(PARI) v=[]; for(m=1, 7500, for(n=1, m-1, if(type(s=(m*n)^2/(m^2-n^2))=="t_INT"&&(s%2), v=concat(v, s)))); vecsort(v, , 8)

CROSSREFS

Cf. A259263, A111200.

Sequence in context: A195239 A122064 A320511 * A325608 A255103 A184542

Adjacent sequences:  A259285 A259286 A259287 * A259289 A259290 A259291

KEYWORD

nonn

AUTHOR

Derek Orr, Jun 23 2015

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 10 10:35 EDT 2021. Contains 342845 sequences. (Running on oeis4.)