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A245388 n such that n - tau(n) is a perfect square. 3
1, 2, 3, 4, 8, 11, 24, 83, 85, 125, 152, 156, 175, 227, 297, 365, 443, 445, 533, 584, 600, 629, 847, 924, 965, 969, 1036, 1091, 1304, 1308, 1458, 1523, 1612, 1685, 1800, 1853, 1960, 2027, 2316, 2340, 2409, 2605, 2716, 2813, 3029, 3251, 3729, 3973, 4108, 4233 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

n - tau(n) = A049820(n) is the number of positive integers < n that do not divide n.

LINKS

Robert Israel, Table of n, a(n) for n = 1..1000

EXAMPLE

4 - tau(4) = 4 - 3 = 1^2 so 4 is in the sequence.

MAPLE

filter:= proc(n) local t;

  t:= numtheory:-tau(n);

  issqr(n-t)

end proc;

select(filter, [$1..10^4]);

MATHEMATICA

Select[Range[10^4], IntegerQ[Sqrt[# - DivisorSigma[0, #]]]&] (* Jean-Fran├žois Alcover, Apr 12 2019 *)

PROG

(Sage)

def is_A245388(n):

    a = sloane.A000005

    return is_square(n - a(n))

A245388_list = lambda up_to: filter(is_A245388, (1..up_to))

A245388_list(4333) # Peter Luschny, Jul 20 2014

CROSSREFS

Cf. A000005, A049820, A245197

Sequence in context: A269797 A280195 A286224 * A164573 A064418 A171164

Adjacent sequences:  A245385 A245386 A245387 * A245389 A245390 A245391

KEYWORD

nonn

AUTHOR

Robert Israel, Jul 20 2014

STATUS

approved

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Last modified November 29 05:53 EST 2020. Contains 338756 sequences. (Running on oeis4.)