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A102836
Composite numbers whose exponents in their canonical factorization lie in the geometric progression 1, 2, 4, ...
2
18, 50, 75, 98, 147, 242, 245, 338, 363, 507, 578, 605, 722, 845, 847, 867, 1058, 1083, 1183, 1445, 1587, 1682, 1805, 1859, 1922, 2023, 2523, 2527, 2645, 2738, 2883, 3179, 3362, 3698, 3703, 3757, 3971, 4107, 4205, 4418, 4693, 4805, 5043, 5547, 5618, 5819
OFFSET
1,1
COMMENTS
The first term not in A095990 is a(70) = 11250.
LINKS
EXAMPLE
Canonical factorization of a(70) = 11250 = 2^1 * 3^2 * 5*4 or 2,3,5 raised to powers 1,2,4 which is a geometric progression.
MATHEMATICA
q[n_] := Module[{e = FactorInteger[n][[;; , 2]]}, Length[e] > 1 && e == 2^Range[0, Length[e]-1]]; Select[Range[6000], q] (* Amiram Eldar, Jun 29 2024 *)
PROG
(PARI) /* Numbers whose factors are primes to perfect powers in a geometric progression. */ geoprog(n, m) = { local(a, x, j, nf, fl=0); for(x=1, n, a=factor(x); nf=omega(x); for(j=1, nf, if(a[j, 2]==2^(j-1), fl=1, fl=0; break); ); if(fl&nf>1, print1(x", ")) ) }
(PARI) is(n) = if(n == 1 || isprime(n), 0, my(e = factor(n)[, 2]); for(i = 1, #e, if(e[i] != 2^(i-1), return(0))); 1); \\ Amiram Eldar, Jun 29 2024
CROSSREFS
Sequence in context: A089219 A102835 A095990 * A217750 A180292 A143928
KEYWORD
easy,nonn
AUTHOR
Cino Hilliard, Feb 27 2005
STATUS
approved