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A104646
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Smallest semiprime equal to the sum of n distinct primes.
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1
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9, 10, 21, 34, 51, 58, 77, 106, 129, 166, 201, 262, 287, 334, 381, 454, 501, 586, 669, 718, 791, 922, 989, 1082, 1167, 1282, 1371, 1486, 1623, 1754, 1851, 2018, 2127, 2326, 2427, 2638, 2747, 2974, 3093, 3274, 3459, 3694, 3831, 4034, 4227, 4534, 4661, 4894, 5123, 5366
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OFFSET
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2,1
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COMMENTS
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The sequence is monotonic for its first million terms. Does there exist n with a(n) < a(n-1)? [Charles R Greathouse IV, Aug 24 2011]
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LINKS
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EXAMPLE
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a(2) = 9 because 9 = 2+7=9 = 3*3;
a(3) = 10 because 10 = 2+3+5 = 2*5;
a(4) = 21 because 21 = 2+3+5+11 = 3*7.
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PROG
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(PARI) issemi(n)=bigomega(n)==2
a(n)={
my(v=primes(n+3), s=sum(i=1, n+1, v[i]), m, t, t1);
m=s+v[n+2]-5;
forstep(i=n+1, 1, -1,
t=s-v[i];
if(t >= m, break);
if(issemi(t), return(t))
);
s+=v[n+2];
m=s+v[n+3]-10;
for(i=2, n+2,
t=s-v[i];
for(j=1, i-1,
t1=t-v[j];
if(t1 >= m, break);
if(issemi(t1), m=t1)
)
);
if(issemi(m), return(m));
error("could not find a("n")")
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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