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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.)