From 5024901379d4a6bc0d2ecc9338701395f85199ba Mon Sep 17 00:00:00 2001
From: Iru Cai <mytbk920423@gmail.com>
Date: Thu, 13 May 2021 20:05:58 +0800
Subject: 684,686,700

---
 euler_686.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 euler_686.py

(limited to 'euler_686.py')

diff --git a/euler_686.py b/euler_686.py
new file mode 100644
index 0000000..01af7b2
--- /dev/null
+++ b/euler_686.py
@@ -0,0 +1,24 @@
+#!/usr/bin/pypy
+
+import math
+
+# calculate the length-l prefix of 2^n
+def prefix_power2(n,l):
+    log = math.log10(2)*n
+    declog = log - math.floor(log)
+    return math.floor(pow(10,declog+l-1))
+
+def main():
+    i = 0
+    count = 0
+    while count < 678910:
+        i = i + 1
+        if prefix_power2(i, 3) == 123:
+            count = count + 1
+            if count % 1000 == 0:
+                print("p({},{})={}".format(123,count,i))
+
+    print("p({},{})={}".format(123,count,i))
+
+if __name__ == "__main__":
+    main()
-- 
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