login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A256545 Composite n such that n*phi(n) is in A002378. 2
6, 30, 434, 510, 616, 912, 1640, 2989, 3003, 5934, 7280, 8600, 10726, 12700, 13825, 14288, 18699, 19389, 54153, 59394, 59906, 70563, 72816, 116052, 117964, 121954, 131070, 134212, 140752, 177000, 206514, 210728, 274023, 319522, 418610, 437736, 456666 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Composite n such that 4*A002618(n)+1 is a square.

For all primes p, 4*A002618(p) + 1 = (2*p-1)^2.

The only semiprime < 10^7 in the sequence is 6.

n=2*p with p prime is in the sequence if 2*p-1 is in A001653.  However, the only such p < 10^3000 is 3.

Similarly, n=3*p with p prime is in the sequence if 2*p-1 is in A080806.  However, the only such p < 10^3000 is 2.

LINKS

Robert Israel, Table of n, a(n) for n = 1..62

EXAMPLE

a(1) = 6 is in the sequence because 6*phi(6) = 12 = 4*3.

MAPLE

select(n -> not isprime(n) and issqr(1+4*n*numtheory:-phi(n)), [$1..10^6]);

MATHEMATICA

Select[Range[10^6], !PrimeQ[#]&&IntegerQ[Sqrt[4*#*EulerPhi[#]+1]]&] (* Ivan N. Ianakiev, Apr 02 2015 *)

PROG

(PARI) lista(nn) = {forcomposite (n=1, nn, if (ispolygonal(n*eulerphi(n)/2, 3), print1(n ", ")); ); } \\ Michel Marcus, Apr 02 2015

CROSSREFS

Cf. A001653, A002378, A002618, A080806.

Sequence in context: A201135 A111876 A119634 * A075591 A130075 A066388

Adjacent sequences:  A256542 A256543 A256544 * A256546 A256547 A256548

KEYWORD

nonn

AUTHOR

Robert Israel, Apr 01 2015

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 4 04:39 EDT 2020. Contains 334815 sequences. (Running on oeis4.)