A231484 Numbers n such that n, sigma(n) and sigma(sigma(n)) are odd.

1, 81, 1476326929, 10994571025, 61436066769, 98551417041, 119582481249, 141447449025, 220545762129, 388895668225, 619568914129, 890560253025, 970952066161, 2580138650961, 3076652813521, 3739640454225, 4138876942929, 4758545225281, 9262289646801, 26494089912081

Offset

1,2

Comments

Contains exactly odd terms of A008848.

Odd n is a term if and only if both n and sigma(n) are perfect squares.

Links

Donovan Johnson, Table of n, a(n) for n = 1..125

Vladimir Letsko Mathematical Marathon, problem 180 (in Russian)

Example

81 is a term because 81, sigma(81) = 121 and sigma(sigma(81)) = 133 are all odd.

Prog

(PARI) forstep(r=1, 5147241, 2, n=r^2; if(sigma(sigma(n))%2<>0, print1(n ", "))) \\ Donovan Johnson, Nov 09 2013
(PARI) forstep(r=1, 5147241, 2, if(!isprime(r)&&issquare(sigma(n=r^2)), print1(n", "))) \\ Charles R Greathouse IV, Nov 12 2013

Crossrefs

Cf. A008848.

Sequence in context: A309110 A116268 A128838 * A340398 A347172 A347175

Adjacent sequences: A231481 A231482 A231483 * A231485 A231486 A231487

Keyword

nonn

Author

Vladimir Letsko, Nov 09 2013

Extensions

a(5) corrected by Donovan Johnson, Nov 09 2013

Status

approved