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A276460 Numbers k such that for any positive integers a < b, if a * b = k then b - a is a square. 1
0, 1, 2, 5, 17, 37, 101, 197, 257, 401, 577, 677, 901, 1297, 1601, 2917, 3137, 4357, 5477, 7057, 8101, 8837, 10001, 12101, 13457, 14401, 15377, 15877, 16901, 17957, 20737, 21317, 22501, 24337, 25601, 28901, 30977, 32401, 33857, 41617, 42437, 44101, 50177, 52901 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

A majority of numbers are primes of form m^2+1 (A002496), and it appears that the composite numbers of the form m^2+1: 901, 10001, 20737, 75077, 234257, 266257, 276677, 571537,... are semiprimes.

For n >1, a(n)==1,5 mod 12 and a(n)==1,5 mod 16.

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..10000

EXAMPLE

901 is in the sequence because 901 = 1*901 = 17*53 => 901-1 = 30^2 and 53-17 = 6^2.

MATHEMATICA

t={}; Do[ds=Divisors[n]; If[EvenQ[Length[ds]], ok=True; k=1; While[k<=Length[ds]/2&&(ok=IntegerQ[Sqrt[Abs[ds[[k]]-ds[[-k]]]]]), k++]; If[ok, AppendTo[t, n]]], {n, 2, 10^5}]; t

PROG

(Python)

from __future__ import division

from sympy import divisors

from gmpy2 import is_square

A276460_list = [0]

for m in range(10**3):

    k = m**2+1

    for d in divisors(k):

        if d > m:

            A276460_list.append(k)

            break

        if not is_square(k//d - d):

            break # Chai Wah Wu, Sep 04 2016

CROSSREFS

Cf. A002496, A134406.

Sequence in context: A078523 A078324 A240322 * A002496 A127436 A064168

Adjacent sequences:  A276457 A276458 A276459 * A276461 A276462 A276463

KEYWORD

nonn

AUTHOR

Michel Lagneau, Sep 03 2016

EXTENSIONS

Terms 0, 1 added by Chai Wah Wu, Sep 04 2016

STATUS

approved

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Last modified September 24 22:27 EDT 2018. Contains 315360 sequences. (Running on oeis4.)