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A069065
Sigma(n)-phi(n) is a triangular number.
1
1, 6, 33, 49, 62, 65, 70, 72, 74, 77, 90, 126, 206, 210, 216, 297, 371, 427, 435, 465, 611, 626, 662, 665, 715, 731, 779, 784, 790, 846, 851, 854, 855, 899, 917, 1147, 1183, 1311, 1322, 1397, 1400, 1464, 1529, 1530, 1560, 1687, 1766, 1781, 1799, 1800, 1810
OFFSET
1,2
LINKS
MATHEMATICA
tnQ[n_]:=IntegerQ[(Sqrt[1+8(DivisorSigma[1, n]-EulerPhi[n])]-1)/2]; Select[ Range[ 2000], tnQ] (* Harvey P. Dale, Jan 08 2014 *)
CROSSREFS
Sequence in context: A132548 A140521 A171141 * A073343 A157872 A153127
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Apr 05 2002
STATUS
approved