login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A069066 Sigma(n)+phi(n) is a triangular number. 5

%I #10 Nov 27 2013 15:32:35

%S 3,5,18,38,94,114,130,168,198,202,222,238,247,314,495,518,531,612,630,

%T 662,722,754,812,914,969,1007,1125,1138,1254,1258,1260,1313,1316,1558,

%U 1665,1736,1778,1794,1957,2110,2138,2233,2279,2327,2360,2364,2385,2474

%N Sigma(n)+phi(n) is a triangular number.

%H Charles R Greathouse IV, <a href="/A069066/b069066.txt">Table of n, a(n) for n = 1..10000</a>

%t triangularQ[n_] := IntegerQ[ Sqrt[8n + 1] ]; Select[ Range[2500], triangularQ[ DivisorSigma[1, #] + EulerPhi[#]] &] (* _Jean-François Alcover_, Jul 17 2013 *)

%o (PARI) is(n)=ispolygonal(sigma(n=factor(n))+eulerphi(n),3) \\ _Charles R Greathouse IV_, Nov 27 2013

%K nonn

%O 1,1

%A _Benoit Cloitre_, Apr 05 2002

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 10:31 EDT 2024. Contains 371240 sequences. (Running on oeis4.)