# Why isn't the naive PRIMES algorithm in P?

The naive algorithm tries dividing $n$ by $2 \dots n-1$ to see if it divides without a remainder. Each division can be done in $O(n)$-time and there are $O(n)$ divisions to be made. What's wrong with this algorithm, or rather why is it not in $P$ complexity?

-

## 1 Answer

Primality testing algorithms have their complexity measured as the number of digits grows to infinity, not the size of the integer itself. Given an integer $N$ with $n$ digits, so that $N\approx 10^n$, the algorithm you gave above grows exponentially in $n$, the number of digits.

-
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 9, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0}, "config": {"markdown_headings": true, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.18, "absolute_threshold": 10, "end_threshold": 15, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "github"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "english_confidence": 0.9398682117462158, "perplexity_flag": "head"}