11
An arithmetico-geometric sequence is the elementwise product of an arithmetic sequence and a geometric sequence. For example, 1 -4 12 -32
is the product of the arithmetic sequence 1 2 3 4
and the geometric sequence 1 -2 4 -8
. The nth term of an integer arithmetico-geometric sequence can be expressed as
$$a_n = r^n \cdot (a_0 + nd)$$
for some real number \$d\$, nonzero real \$r\$, and integer \$a_0\$. Note that \$r\$ and \$d\$ are not necessarily integers.
For example, the sequence 2 11 36 100 256 624 1472 3392
has \$a_0 = 2\$, \$r = 2\$, and \$d = 3.5\$.
Input
An ordered list of \$n \ge 2\$ integers as input in any reasonable format. Since some definitions of geometric sequence allow \$r=0\$ and define \$0^0 = 1\$, whether an input is an arithmetico-geometric sequence will not depend on whether \$r\$ is allowed to be 0. For example, 123 0 0 0 0
will not occur as input.
Output
Whether it is an arithmetico-geometric sequence. Output a truthy/falsy value, or two different consistent values.
Test cases
True:
1 -4 12 -32
0 0 0
-192 0 432 -1296 2916 -5832 10935 -19683
2 11 36 100 256 624 1472 3392
-4374 729 972 567 270 117 48 19
24601 1337 42
0 -2718
-1 -1 0 4 16
2 4 8 16 32 64
2 3 4 5 6 7
0 2 8 24
False:
4 8 15 16 23 42
3 1 4 1
24601 42 1337
0 0 0 1
0 0 1 0 0
1 -1 0 4 16
1FYI you can use inline math mode with
\$
to write things like $ a_{0} $. – FryAmTheEggman – 2018-11-20T02:28:47.690Are two-term inputs indeed possible? There aren't any in the test cases. – xnor – 2018-11-20T03:40:12.537
@xnor Trivially you can set $r=1$ or $d=0$ so the sequences aren't unique in that case, but the output should always be truthy – Giuseppe – 2018-11-20T03:55:58.370
@user202729 Thanks – lirtosiast – 2018-11-20T06:19:54.817
1Suggest testcase 0 2 8 24, 0 0 1, 0 0 0 1 – tsh – 2018-11-20T12:12:49.783
1
1 -1 0 4 16
would be a useful False case, since it shares four consecutive elements with each of the True cases1 -1 0 4 -16
and-1 -1 0 4 16
. – Anders Kaseorg – 2018-11-20T22:33:29.810