A136113 - OEIS

A136113

Pentagonal numbers > 0 which are not the difference of two larger pentagonal numbers.

1, 5, 12, 35, 51, 92, 117, 176, 330, 477, 782, 852, 1080, 3876, 4347, 7526, 7740, 9801, 13776, 14652, 22632, 24512, 27270, 39285, 69876, 85562, 88452, 103622, 124272, 137562, 144926, 193142, 220992, 268182, 315792, 343922, 354051, 403782, 523626

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..500

FORMULA

a(n)=A000326(A136112(n)). A number m is in this sequence iff A136114(m) = 0 iff A136115(m) = 0.

EXAMPLE

a(1..3)=P(1),P(2),P(3) since these cannot be written as difference of 2 other pentagonal numbers > 0.

P(4)=22=P(8)-P(7), therefore P(4) is not in this sequence.

PROG

(PARI) P(n)=n*(3*n-1)>>1 /* a.k.a. A000326 */

isPent(t)=P(sqrtint((t<<1)\3)+1)==t

for( i=1, 999, for( j=i+1, (P(i)-1)\3, isPent(P(i)+P(j))&next(2)); print1(P(i)", "))

CROSSREFS

Cf. A000326, A136112-A136118.

Sequence in context: A333886 A192243 A292104 * A298992 A050189 A308344

Adjacent sequences: A136110 A136111 A136112 * A136114 A136115 A136116

KEYWORD

nonn

AUTHOR

M. F. Hasler, Dec 15 2007, Feb 07 2008

EXTENSIONS

a(34)-a(39) from Donovan Johnson, Sep 05 2008

STATUS

approved