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A076771 Even numbers n representable as the sum of two non-coprime odd composites. 1
18, 24, 30, 36, 40, 42, 48, 50, 54, 56, 60, 66, 70, 72, 78, 80, 84, 88, 90, 96, 98, 100, 102, 104, 108, 110, 112, 114, 120, 126, 130, 132, 136, 138, 140, 144, 150, 152, 154, 156, 160, 162, 168, 170, 174, 176, 180, 182, 184, 186, 190, 192, 196, 198, 200, 204 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

It is conjectured that there is no N such that for all n > N, every even number is represented.

The conjecture is true since 2p, p prime, is never a member of this sequence. - Charles R Greathouse IV, Aug 08 2011

An equivalent definition of this sequence: even numbers n such that phi(n) < (n-4)/2. - Arkadiusz Wesolowski, Aug 08 2011

Also: products of an even number >= 6 and an odd number >= 3. - Charles R Greathouse IV, Aug 08 2011

LINKS

Arkadiusz Wesolowski, Table of n, a(n) for n = 1..1000

EXAMPLE

40 = 5*(3 + 5).

PROG

(PARI) upTo(lim)=my(v=List()); forstep(a=6, lim\3, 2, forstep(b=3, lim\a, 2, listput(v, a*b))); vecsort(Vec(v), , 8) \\ Charles R Greathouse IV, Aug 08 2011

(PARI) upTo(lim)=my(v=List(), p=7, m); forprime(q=8, lim\2, forstep(n=p+2, q-2, 2, m=n; while((m<<=1)<=lim, listput(v, m))); p=q); forstep(n=2*p+4, lim, 4, listput(v, n)); forprime(p=3, lim>>3, m=p<<3; while(m<=lim, listput(v, m); m<<=1)); vecsort(Vec(v)) \\ Charles R Greathouse IV, Aug 08 2011

CROSSREFS

Cf. A161962.

Sequence in context: A076770 A105679 A160810 * A179014 A105093 A334015

Adjacent sequences: A076768 A076769 A076770 * A076772 A076773 A076774

KEYWORD

nonn,easy

AUTHOR

Jon Perry, Nov 14 2002

EXTENSIONS

Offset corrected by Arkadiusz Wesolowski, Aug 08 2011

STATUS

approved

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Last modified January 31 04:27 EST 2023. Contains 359947 sequences. (Running on oeis4.)