1 files changed, 24 insertions, 0 deletions
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()