591 is the 6th term.
Formula for nth term is 1-7n+15n²-8n³+2n⁴, where n≥0, so the first term is when n=0. The 6th term is when n=5. This formula gives 1363 as the 7th term.
Other formulas may also work because only a few terms have been given. These other formulas are likely to predict a different 6th term.